Improves using HttpClient

[bartizan]

Improve using of HttpClient.

The preferred and recommended approach is to manage HttpClient lifetimes using IHttpClientFactory via Dependency Injection (DI).

Note: ApiCenterClient is left untouched.

[bartizan]

@waldekmastykarz
Could ApiCenterClient and related handlers be moved to Abstractions project?

[waldekmastykarz]

Could ApiCenterClient and related handlers be moved to Abstractions project?

They shouldn't, because they belong to a specific group of plugins.