[REQ] Allow extensibility for C# clients

Is your feature request related to a problem? Please describe.

We're having trouble using the generated .NET clients. It seems they are not open to extension like other client libraries. Specifically, all abstractions (Configuration, IApiAccessor, etc.) are generated within the client rather than factored out to a common library that extensions can rely on.

Describe the solution you'd like

• Publish an official library for base abstractions that generated client libraries depend on.
• Support a client constructor that accepts an HttpClient so that the official client extensibility points can be leveraged.

Describe alternatives you've considered

From a first principles perspective, I don't think other solutions are possible without providing base abstractions. We will likely evaluate other generators or write our own.

Additional context

Providing extensibility would close #3581.

Start by reviewing how generated .NET clients define Configuration and IApiAccessor, then compare their constructor surface with the linked HttpClient extensibility guidance. Review the discussion and related issue #3581 before defining scope; done means generated clients can rely on official shared abstractions and accept an HttpClient.