Add better support for topic validation in nemesis changefeed tests

[aerfrei]

Right now in nemesis tests when we test changefeeds with the full_table_name option we assume that for a table {foo}, the full table name will be d.public.{foo}. That is to say the values d and public are hardcoded. We should be able to support other values for the db and schema names.

Additionally, we do not test behavior when the user specifies a topic name.

Describe the solution you'd like
If there were a way to both run the test with dbs and schemas with different names and to fetch those at assertion time, that would be ideal.
Also, we should be able to specify a topic name at changefeed creation time and test that it is respected in the topic name validator.

Jira issue: CRDB-46481

[blathers-crl]

cc @cockroachdb/cdc

[wenyihu6]

For these new sub-issues, you can link the parent epic to https://cockroachlabs.atlassian.net/browse/CRDB-38724.

[aerfrei]

@wenyihu6 I'm having trouble connecting it directly, maybe because that's an issue and not an epic. I'm adding it to that issue's parent for now: https://cockroachlabs.atlassian.net/browse/CRDB-42866. Do you know if there's another way to add those to that issue?

[wenyihu6]

Ahh, I didn't realize that the randomized testing one is not an epic. I think what you did is fine. Thanks!