build: Support manually generating C# bindings via Github action and add a README

[dekomissMSFT]

Update the Github action that generates C# bindings for Regorus to support manual generation. This allows for easily generating Nuget packages for any branch, which is useful for integration testing of in-progress Regorus changes with other projects during development.

Also update the Readme for the C# bindings to explain how to generate a nuget package.

[anakrish]

@dekomissMSFT The clippy errors are unrelated to your changes. The rusttool chain has moved ahead and clippy is now flagging more code to rewrite.
I have a PR to fix those: #424

[anakrish]

@dekomissMSFT The PR looks good. Please squash your commits and fix the commit message.

I've been also following the conventional commit approach of tagging the commit message. Then the commits will be grouped automatically in the release description: https://github.com/microsoft/regorus/releases

[dekomissMSFT]

@dekomissMSFT The PR looks good. Please squash your commits and fix the commit message.

I've been also following the conventional commit approach of tagging the commit message. Then the commits will be grouped automatically in the release description: https://github.com/microsoft/regorus/releases

Fixed

[anakrish]

LGTM