changefeedccl: Db-level changefeed: implement ALTER DEFAULT PRIVILEGES GRANT CHANGEFEED on DATABASES

[KeithCh]

Add default privileges for changefeeds on databases.

Jira issue: CRDB-53702
Epic: CRDB-1421

[blathers-crl]

Hi @KeithCh, please add a C-ategory label to your issue. Check out the label system docs.

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[blathers-crl]

cc @cockroachdb/cdc