feat(client): add http.RoundTripper Middleware Configuration Option to Client

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

Added a configuration option for the client that supports an http.RoundTripper middleware, making request handling easy to customise.

This change opens up possibilities for various DevOps enhancements to the users of this client,, such as adding monitoring metrics around HTTP requests towards GitLab API calls.