feat: add glue namespace impl spec (#146)

This PR adds a design for AWS Glue Data Catalog support in Lance
namespaces.

Author: geruh

docs/src/spec/impls/glue.md

@@ -0,0 +1,28 @@
+# Lance Glue Namespace
+
+**Lance Glue Namespace** is an implementation using AWS Glue Data Catalog.
+For more details about AWS Glue, please read the [AWS Glue Data Catalog Documentation](https://docs.aws.amazon.com/glue/).
+
+## Namespace Mapping
+
+An AWS Glue Data Catalog can be viewed as the root Lance namespace.
+A database in Glue maps to the first level Lance namespace,
+to form a 2-level Lance namespace as a whole.
+
+## Table Definition
+
+When fully implemented, a Lance table should appear as a [Table](https://docs.aws.amazon.com/glue/latest/webapi/API_Table.html) 
+object in AWS Glue with the following requirements:
+
+1. the [`TableType`](https://docs.aws.amazon.com/glue/latest/webapi/API_Table.html#Glue-Type-Table-TableType) must be set to `EXTERNAL_TABLE` to indicate this is not a Glue managed table
+2. the [`StorageDescriptor.Location`](https://docs.aws.amazon.com/glue/latest/webapi/API_StorageDescriptor.html#Glue-Type-StorageDescriptor-Location) must point to the root location of the Lance table
+3. the [`Parameters`](https://docs.aws.amazon.com/glue/latest/webapi/API_Table.html#Glue-Type-Table-Parameters) must follow:
+    1. there is a key `table_type` set to `lance` (case insensitive)
+    2. there is a key `managed_by` set to either `storage` or `impl` (case insensitive). If not set, default to `storage`
+    3. there is a key `version` set to the latest numeric version number of the table. This field will only be respected if `managed_by=impl`
+
+## Requirement for Implementation Managed Table
+
+Updates to implementation-managed Lance tables must use AWS Glue’s `VersionId` for conditional updates through the
+[UpdateTable](https://docs.aws.amazon.com/glue/latest/webapi/API_UpdateTable.html) API. If the `VersionId` does not 
+match the expected version, the update fails to prevent concurrent modification conflicts.