storage: do not generate timestamp bindings for CDCv2 sources

[petrosagg]

Motivation

Fix MaterializeInc/database-issues#3355. CDCv2 sources don't need bindings since their timestamps are completely governed by the actual data stream.

Testing

• This PR has adequate test coverage / QA involvement has been duly considered.

Release notes

This PR includes the following user-facing behavior changes:

[frankmcsherry]

Seems legit! I don't understand if this wedges CDCv2 or leaves it in a healthy state, but the implementation does seem to better conform to the documentation!

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