Add StrEnum.Npgsql package

[dmytro-khmara]

Summary

• Introduces StrEnum.Npgsql, an EF Core + Npgsql integration that lets StrEnum string enums be stored either as text or as native Postgres enum types (analogous to MapEnum for regular C# enums).
• Public API: UseStringEnums() on the options builder; HasPostgresStringEnum<T>() / MapStringEnumAsPostgresEnum<T>() on the model builder; HasPostgresStringEnum<T>() on the property builder.
• Self-contained: depends only on StrEnum and Npgsql.EntityFrameworkCore.PostgreSQL. The value converter selector logic is inlined rather than pulled from StrEnum.EntityFrameworkCore.
• Targets net6.0–net10.0 with the matching EFCore.PG version range per TFM. Mirrors the sibling repos: Dockerfile, build/publish workflows, .gitignore, .dockerignore, LICENSE.txt, README, sln.

Test plan

• dotnet build succeeds for all five target frameworks
• dotnet test — 18/18 pass on net10.0 (xUnit + FluentAssertions)
• Smoke-test against a real Postgres instance (text and PG-enum modes, including a dotnet ef migrations add to verify the generated migration shape)
• Verify docker build --target test succeeds in CI