Summary

Adds relational database transports for SQL Server and PostgreSQL.

At a high level this PR:

• turns SlimMessageBus.Host.Sql into a working SQL Server transport provider
• adds a new SlimMessageBus.Host.PostgreSql transport provider
• implements queue and durable topic/subscription flows backed by relational tables
• adds sequential/client GUID id generation options for PK-friendly message IDs
• adds provider-style configuration extensions matching the existing transport plugins
• adds topology provisioning, polling consumers, error handling, and repository layers
• adds real-container integration tests for SQL Server and PostgreSQL
• adds relational transport BenchmarkDotNet benchmarks
• updates docs, solution wiring, CI matrix entries, and packaging scripts

Validation

• dotnet test src\Tests\SlimMessageBus.Host.Sql.Test\SlimMessageBus.Host.Sql.Test.csproj -v minimal
• dotnet test src\Tests\SlimMessageBus.Host.PostgreSql.Test\SlimMessageBus.Host.PostgreSql.Test.csproj -v minimal
• git diff --check

Both provider integration suites pass against real Testcontainers databases.