cdc: alter changefeed to add a table can hit boundary timestamps from before the alter statement

See https://github.com/cockroachlabs/support/issues/2127

ALTER CHANGEFEED to add a table to an existing changefeed should add it as of the ALTER statement, and therefore should not hit boundaries from before that timestamp.

Jira issue: CRDB-25676

Start by reproducing the ALTER CHANGEFEED scenario described in support issue 2127, then trace the changefeed ALTER handling and timestamp-boundary behavior. Done means a newly added table begins at the ALTER statement timestamp and does not encounter boundaries from before that timestamp.

Written by the indexing model from the issue text.