Add ability to change default host

For the testing purposes I need to change `https://api.github.com` host to the mock API url, but there is no way to do it. All auto-tests should go to the mock service, instead of real GitHub.

We can use enterprise url as a workaround, but it looks like a wrong path.

I see 3 ways to implement this feature:

1. Check in `Github\Client::__construct` method if `AddHostPlugin` already defined, and skip it's definition in this case. Set enterprise URL will require the same check.

2. Add method to change host plugin manually in `Github\Client` object.

3. Make `Github\Client::getHttpClientBuilder` public.

Start at Github\Client::__construct and Github\Client::getHttpClientBuilder, then inspect how AddHostPlugin and the enterprise URL are configured. Decide how callers can set a mock host without sending requests to real GitHub, and verify that the client’s requests use the configured host.

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