[WIP]C#: Add generic commands

[MikeMwita]

Issue link

This Pull Request is linked to issue (URL): [REPLACE ME]

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• This Pull Request is related to one issue.
• Commit message has a detailed description of what changed and why.
• Tests are added or updated.
• CHANGELOG.md and documentation files are updated.
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[MikeMwita]

@Yury-Fridlyand, is there FFI support for generic commands atm, so I don't have to rely on CustomCommand & thus switch to FFI bindings.

[yipin-chen]

@Yury-Fridlyand, is there FFI support for generic commands atm, so I don't have to rely on CustomCommand & thus switch to FFI bindings.

Hi Mike,
Currently, we are busy working on 2.0 GA release, and Yury might be delayed in replying or reviewing your PR.
For C#, you can find our progress and implemented support in epic issue #216.

[Yury-Fridlyand]

Using binding generator is TBD. We can use csbindgen - it is pretty simple to use it, but some code changed are need to adopt it.
It produces unsafe code, I wanted to avoid this, that's the reason it is not done yet.

Please don't use custom command. So far I manually copied enum values for request type.