Merge client headers with MP REST Client

Current behavior from MP REST Client is that the default implementation of `ClientHeadersFactory` does not include any headers that might have been specified with `@ClientHeaderParam` or `@HeaderParam`.

This is quite unintuitive for anyone that just adds `@RegisterClientHeaders` onto their interface and expects both propagation and specific headers.

I think it would good if we did that merging outside the default factory, when we know it's the default factory being executed.