feat(sqlserver): Part 5 — catalog introspection (get_relation, metadata module)

[axellpadilla]

Part of dbt-labs#15714 (upstream feature request). Depends on #1. Independent of #4, #6, #7 (can run in parallel).

Work items

• src/metadata/get_relation.rs — AdapterType::SqlServer => sqlserver_get_relation(...) arm
• New src/metadata/sqlserver/mod.rs, modeled on src/metadata/fabric/mod.rs (346 lines), cross-checked against v1's catalog.sql/metadata.sql for anything Fabric's module doesn't need but classic SQL Server does (index metadata via sys.indexes/sys.index_columns)

Full detail: dbt-sqlserver-v2-roadmap plan/02-implementation-steps.md §5.5 ("Catalog introspection")

PRs against sqlserver-v2-port in this repo, use Closes #<this issue>.