[csharp][netcore-httpclient] Reuse HttpClient, Allow use of external HttpClient, Fix Socket Exhaustion, Alternative With Constructor Injection

Created by: Blackclaws

Fixed an existing issue that made the HttpClient open new sockets for each connection even if being reused as the UserAgent was being set on the Client and not the request. Also makes HttpClient settable from external. Fixes: https://github.com/OpenAPITools/openapi-generator/issues/9060

Alternative to https://github.com/OpenAPITools/openapi-generator/pull/9080

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