lib_rpc_http: Expose RetryClient

This simple MR aims to expose the RetryClient of the lib_rpc_http library. This is useful in the context of %Non-Blocking RPCs and especially !9490 (merged) to redirect client RPCs to a socket.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR