Move unified validation APIs to separate package

[captainsafia]

To support using the unified resolver APIs in more scenarios, we would like to move them to a new package instead of the Http.Abstractions package. To do this, we must:

1. Create a new src/Validation directory that contains a Microsoft.Extensions.Validation project.
2. Move the code from https://github.com/dotnet/aspnetcore/tree/main/src/Http/Http.Abstractions/src/Validation into the new directory under src/Validation/src.
3. Move the code from https://github.com/dotnet/aspnetcore/tree/main/src/Http/Http.Abstractions/test/Validation into a new directory under src/Validation/test/Microsoft.Extensions.Validation.Tests.
4. Move the code from https://github.com/dotnet/aspnetcore/tree/main/src/Http/Http.Extensions/gen/Microsoft.AspNetCore.Http.ValidationsGenerator into a new directory under src/Validation/gen.
5. Move the tests from https://github.com/dotnet/aspnetcore/tree/main/src/Http/Http.Extensions/test/ValidationsGenerator into a new directory under src/Validation/test/Micorosft.Extensions.Validation.ValidationsGenerator tests.
6. Update eng/ProjectReferences.props to reference the newly created project.
7. Create a new src/Validation/Validations.slnf file for the project.