MP Rest Client 2.0: Use CDI-managed providers

In MP Rest Client issue 256 (implemented by PR 277, implementations are required to check (1) if CDI exists in the runtime and (2) if CDI is managing an instance of the provider class specified by the client interface - and if so, then use the CDI-managed provider instead of creating one using newInstance(). This applies for cases where the provider class is registered, not a pre-created instance of the provider.