cdc: provide a warning when creating a changefeed with a cursor that's too far in the past

[daniel-crlabs]

Is your feature request related to a problem? Please describe.

Currently customers can create changefeeds specifying the cursor option so it starts from that given time. Unfortunately customers might make typos when entering the cursor epoch time and start a cursor with a timestamp that's too far in the past.

This typo could cause the changefeed to lag very far behind if the table is huge and the customer is unaware of this typo.

Describe the solution you'd like

Provide a warning when the customer creates a changefeed to let them know they are creating a changefeed with a cursor that's too far in the past (maybe if the curosr is older than 5 hours from the time the changefeed is being created) and this could cause changefeed latency.

Describe alternatives you've considered

None

Additional context

Customers might attribute the changefeed latency to many other issues and waste time troubleshooting something that's not the real problem or even recognize/realize the problem was caused because they made a typo.

Jira issue: CRDB-38811

[blathers-crl]

cc @cockroachdb/cdc