[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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