[Cloudtest] Allow setting additional headers in cloudtest requests

[rjobanp]

Motivation

Allows passing in headers for requests made via cloudtest 'controllers'. Necessary for setting new X-MZ-Api-Version header planned to be introduced in PR for https://github.com/MaterializeInc/cloud/issues/7429

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[nrainer-materialize]

Thank you