Rekhoff/csharp default field values (#3235)

# Description of Changes

This is the implementation of issue #3191. This adds a Default attribute
to C# module fields.

**Note**: In C#, attribute arguments must be compile-time constants,
which means you can't directly use non-constant expressions like new
expressions, method calls, or dynamic values in attribute constructors.
(Ref:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes#2324-attribute-parameter-types)

For this reason, these default values are limited to primitive types,
enums, and strings.
This includes (shown as `C# Type` (`BSATN type`):

* `bool` (`Bool`)
* `sbyte` (`I8`)
* `byte` (`U8`)
* `short` (`I16`)
* `ushort` (`U16`)
* `int` (`I32`)
* `unit` (`U32`)
* `long` (`I64`)
* `ulong` (`U64`)
* `float` (`F32`)
* `double` (`F64`)
* `enum` (`Enum`)
* `string` (`String`)
* `null` (`RefOption`) <- Nullable type

Because of C# limitations, for nullable and complex data types, such as
a struct, can take use `[Default(null)]` to populate values with null
defaults. This allows things like structs to workaround the non-constant
expressions in attribute constructors limitation by allowing these
complex types to still be able to be added as new column tables.

The `int` type can also be in the form of Hex or Binary literals, such
as `[Default(0x2A)]` or `[Default(0b00101010)]`

Both Decimal (like `[Default(3.14m)]`) and Char (like `[Default('A')]`)
are unsupported types in BSATN and will still return `BSATN0001` errors.

# API and ABI breaking changes

Not API breaking.
This change only adds the `[Default(value)]` attribute logic.
Using the `[Default(value)]` attribute with older versions SpacetimeDB
C# modules will result in an error.

# Expected complexity level and risk

2

# Testing

Local testing of this requires use of CLI changes in #3278 

- [x] Regression test of functionality added.

Author: rekhoff

crates/bindings-csharp/Codegen.Tests/fixtures/diag/Lib.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel;
 using SpacetimeDB;
 
 public enum LocalEnum { }
@@ -361,6 +362,80 @@ public static partial class InAnotherNamespace
     public partial struct TestDuplicateTableName { }
 }
 
+[SpacetimeDB.Table]
+public partial struct TestDefaultFieldValues
+{
+    [Unique]
+    public int? UniqueField;
+
+    [Default("A default string set by attribute")]
+    public string DefaultString = "";
+
+    [Default(true)]
+    public bool DefaultBool = false;
+
+    [Default((sbyte)2)]
+    public sbyte DefaultI8 = 1;
+
+    [Default((byte)2)]
+    public byte DefaultU8 = 1;
+
+    [Default((short)2)]
+    public short DefaultI16 = 1;
+
+    [Default((ushort)2)]
+    public ushort DefaultU16 = 1;
+
+    [Default(2)]
+    public int DefaultI32 = 1;
+
+    [Default(2U)]
+    public uint DefaultU32 = 1U;
+
+    [Default(2L)]
+    public long DefaultI64 = 1L;
+
+    [Default(2UL)]
+    public ulong DefaultU64 = 1UL;
+
+    [Default(0x02)]
+    public int DefaultHex = 1;
+
+    [Default(0b00000010)]
+    public int DefaultBin = 1;
+
+    [Default(2.0f)]
+    public float DefaultF32 = 1.0f;
+
+    [Default(2.0)]
+    public double DefaultF64 = 1.0;
+
+    [Default(MyEnum.SetByAttribute)]
+    public MyEnum DefaultEnum = MyEnum.SetByInitalization;
+
+    [Default(null!)]
+    public MyStruct? DefaultNull = new MyStruct(1);
+}
+
+[SpacetimeDB.Type]
+public enum MyEnum
+{
+    Default,
+    SetByInitalization,
+    SetByAttribute,
+}
+
+[SpacetimeDB.Type]
+public partial struct MyStruct
+{
+    public int x;
+
+    public MyStruct(int x)
+    {
+        this.x = x;
+    }
+}
+
 [SpacetimeDB.Table]
 [SpacetimeDB.Index.BTree(Name = "TestIndexWithoutColumns")]
 [SpacetimeDB.Index.BTree(Name = "TestIndexWithEmptyColumns", Columns = [])]

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/ExtraCompilationErrors.verified.txt

@@ -92,10 +92,33 @@ var ___hashUnsupportedEnum = UnsupportedEnum.GetHashCode();
     }
   },
   {/*
+[SpacetimeDB.Table]
+public partial struct TestDefaultFieldValues
+                      ^^^^^^^^^^^^^^^^^^^^^^
+{
+*/
+    Message: A 'struct' with field initializers must include an explicitly declared constructor.,
+    Severity: Error,
+    Descriptor: {
+      Id: CS8983,
+      Title: ,
+      HelpLink: https://msdn.microsoft.com/query/roslyn.query?appId=roslyn&k=k(CS8983),
+      MessageFormat: A 'struct' with field initializers must include an explicitly declared constructor.,
+      Category: Compiler,
+      DefaultSeverity: Error,
+      IsEnabledByDefault: true,
+      CustomTags: [
+        Compiler,
+        Telemetry,
+        NotConfigurable
+      ]
+    }
+  },
+  {/*
 SpacetimeDB.Internal.Module.RegisterClientVisibilityFilter(global::Module.MY_SECOND_FILTER);
 SpacetimeDB.Internal.Module.RegisterClientVisibilityFilter(global::Module.MY_THIRD_FILTER);
                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    }
+        {
 */
     Message: Argument 1: cannot convert from 'string' to 'SpacetimeDB.Filter',
     Severity: Error,