[csharp] Use HttpClientFactory for RestSharp clients

[alec-petersen]

Still in progress on this one. Goal is to fix the following issues.

Fix #13961 Fix #14680

Currently, we create a new RestClient for every request, which also creates a new HttpClient for every request. This is known to be an issue (even if you are disposing of the client each time).

This solution adds a HttpClientFactory to cache instances of HttpClient based on BaseUrl (with a separate cache per ApiClient). Lots of considerations to take into account with this change and I'm still sifting through and accounting for the side-effects. The best solution may be to allow the user to provide an IHttpClient factory.

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