materialize tests: Add balancerd+webhooks using testdrive

[ ] 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:

Mar 05 '24 23:03 def-

This is now blocked by https://github.com/MaterializeInc/database-issues/issues/7452, panics reliably.

Mar 06 '24 16:03 def-

This is now part of https://github.com/MaterializeInc/materialize/pull/26291, so can be closed when that merges.

Mar 28 '24 08:03 def-