Add PostgreSQL support object migrations

[MelbourneDeveloper]

Summary

• Add declarative YAML, diff, DDL, and inspection support for PostgreSQL roles, functions, grants, and RLS policies.
• Add PostgreSQL support-object tests, real Postgres E2E coverage, readback coverage, and RLS spec notes.
• Bump DataProviderMigrate to 0.9.8-beta.

Verification

• dotnet csharpier check .
• dotnet test Migration/Nimblesite.DataProvider.Migration.Tests/Nimblesite.DataProvider.Migration.Tests.csproj --configuration Release --no-restore
• dotnet pack Migration/DataProviderMigrate/DataProviderMigrate.csproj --configuration Release --no-restore --output /tmp/dataprovider-pack-check

Closes #33
Closes #34
Closes #35