Make GrpcClientProvider accessible in TestKit

[aklikic]

For now GrpcClientProvider is not accessible in TestKit like HttpClientProvider is.
We need it for creating a client for an external running service (for example mocked by wiremock).
For now you can only access getGrpcEndpointClient that can be used only for the service that is being tested,

[johanandren]

This is not to access the provider but to provide/inject a mock/dummy client for some service called from a component being tested, right?

[aklikic]

Yes