[Feature Request]: ResGen.exe should have an option to generate partial classes

### Summary

There should be a way for `ResXFileCodeGenerator`/`PublicResXFileCodeGenerator` (Resgen.exe) to generate a class with a `partial` modifier/keyword.

### Background and Motivation

Currently when generating Resource class the partial modifier is not included and there is no option to include the partial modifier when generating a class from the `.resx` files.
This prevents one from easily extending the functionality.

I think this is the same as dotnet/core#2943 which appears to have been closed as it was filed in the wrong repo.

### Proposed Feature

Add a `/partialClass` cli-flag similar to the existing `/publicClass` cli-flag,
setting this flag should cause the `partial` keyword to be included in the generated class declaration.
Add variant of `ResXFileCodeGenerator` and `PublicResXFileCodeGenerator` like `PartialResXFileCodeGenerator`and `PublicpartialResXFileCodeGenerator`.

### Alternative Designs

_No response_