Add cargo ci dlls command for building C# DLLs and NuGet packages (#4033)

# Description of Changes
* Added a new `cargo ci dlls` subcommand to build/pack the in-repo C#
NuGet packages and the C# SDK.
* `cargo ci dlls` restores `sdks/csharp/SpacetimeDB.ClientSDK.csproj`
using the freshly built local package outputs as to populate
`sdks/csharp/packages/**`.
* Added a Unity `.meta` skeleton under
`sdks/csharp/unity-meta-skeleton~/**` and overlays those `.meta` files
onto the latest restored versioned package directory to keep Unity GUIDs
stable and import settings consistent.
* Unity-specific import fixes are captured in the skeleton overlay
(notably: preventing Unity from importing incompatible TFMs like
`net8.0`, and marking analyzer DLLs with the `RoslynAnalyzer` label so
Unity can recognize them).
# How to use (local)
```bash
# Build/pack + restore local packages into sdks/csharp/packages/**
cargo ci dlls
```
# API and ABI breaking changes
N/A
# Expected complexity level and risk
2 - Local developer tooling + file overlay into restore output; no
runtime/SDK API behavior changes.
# Testing
- [x] `cargo check -p ci`
- [x] Ran `cargo ci dlls` and verified the output under
`sdks/csharp/packages/**` and the various NuGet package locations.
- [x] Tested a Unity project importing the SpacetimeDB SDK after
generating output and confirmed no errors.

---------

Signed-off-by: Ryan <r.ekhoff@clockworklabs.io>
Co-authored-by: Zeke Foppa <196249+bfops@users.noreply.github.com>
Co-authored-by: John Detter <4099508+jdetter@users.noreply.github.com>

Author: 3 people

sdks/csharp/.meta-check-ignore

@@ -0,0 +1,2 @@
+unity-meta-skeleton~
+unity-meta-skeleton~/**

sdks/csharp/packages/.gitignore

@@ -1,11 +1,15 @@
 # Ignore most of NuGet package structure, except DLLs which are required by Unity.
+/*/*
+!/*/*/
 /*/*/*
 
 !/*/*/analyzers
 !/*/*/analyzers.meta
+!/*/*/analyzers/**
 
 !/*/*/lib
 !/*/*/lib.meta
+!/*/*/lib/**
 
 # Ignore XML documentation metadata from packages too.
 *.xml