[csharp] public TokenProvider to make it implementable from consumer projects

[daberni]

TokenProvider.GetAsync() was declared with visibility modifier internal. This resulted in a generated class which could only be implemented in the same module, although the class itself is declared as public making it accessible globally.

In the use case of providing an SDK where the Token is provided from some external source this would not be supported right now. By declaring GetAsync() public, anyone can implement its own TokenProvider and register it in the IServiceCollection for automatic dependency lookup.

At the same point I cleared up the unclear declaration of a _token collection, which only is used from RateLimitProvider. RateLimitProvider now only uses the TokenContainer directly, without roundtrip of TokenProvider._tokens.

Any other implementatio nof TokenProvider could declare its own list if necessary.

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@mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

[daberni]

@wing328 it looks like the suggested technical committee members are quite inactive at the moment. What would be the best way to still be able to contribute to these projects when there is no one approving the pull requests?

[devhl-labs]

I believe this was already fixed by #22233 with small changes pending in #22796.

[daberni]

You are right, rebased the branch and there are no significant changes remaining 👍