cdc: allow schema as a changefeed target

Some users would like to create changefeeds that watch all (or most) tables in an entire schema. Syntax could be like the following:

```
CREATE CHANGEFEED FOR SCHEMA schema
```

There may be some tables in the schema that users do not want watched by the changefeed. We would need to add syntax for an exclusion list (either explicit or with regex support).

This builds on database targets for changefeeds in https://github.com/cockroachdb/cockroach/issues/93793.

Jira issue: CRDB-42913

Epic CRDB-39384