cdc: warn when adding a column family to a table with a running changefeed

[HonoreDB]

With links to docs.

Jira issue: CRDB-13047

Epic CRDB-31637

[blathers-crl]

cc @cockroachdb/cdc

[miretskiy]

What's the motivation here? Not sure how this would play w/ tying jobs system into schema changes...

[ajwerner]

Regarding this, one thing I've been thinking is it's now very easy to breaking the rangefeed.enabled setting down per table (#70614, #73045)

I wonder if, in some future, we'd want to go an make users mark tables as explicitly having changefeeds enabled. Then we could warn if adding a column family to a table which has changefeeds enabled. cc @amruss

[miretskiy]

Adding this issue for sql-foundations triage.
Schema changes could be disruptive to running changefeeds because some of those changes
trigger backfills. This often causes "surprises" that our customers would like to avoid (including
overloading their downstream clusters, causing significant latency spikes, etc).

We would like to have a guard-rail to ensure that when schema change is attempted, the user
will either get a warning, or even an error message (similar to how "schema_locked" table parameter operates)
to get an explicit confirmation.

[rafiss]

@dikshant will determine how to prioritize this.

[dikshant]

Internal discussion here: https://cockroachlabs.slack.com/archives/C9TGBJB44/p1695330349551529
We won't be investing in this right now since there are other priorities for schema: eg finishing declarative schema changer.