cdc: fully support key_column with CDC query

[wenyihu6]

There are some edge cases that custom keys do not support. This issue is opened to discuss/track the work to decide if we should support and the work needed to be done to support.

After some testing, these are things that work/don't work with custom key columns so far: TLDR: it has to be an actual column that exists on the table - not something created in CDC queries or created afterwards.

1. using a key column that user created in CDC queries

   CREATE CHANGEFEED WITH key_column='double_b', unordered AS SELECT concat(b, c) AS double_b FROM foo

2. selecting on a virtual column does not work

   CREATE TABLE foo (a INT PRIMARY KEY, b STRING, c STRING, d STRING AS (concat(b, c)) VIRTUAL)
   CREATE CHANGEFEED WITH key_column='d', unordered AS SELECT d FROM foo 

3. selecting on a hidden column works

   CREATE TABLE foo (a INT PRIMARY KEY, b STRING NOT VISIBLE)
   CREATE CHANGEFEED WITH key_column='b', unordered AS SELECT b FROM foo

4. selecting on cdc_prev or cdc_is_delete doesn't seem to work, and cdc_is_delete is not recognized as a function either when used in CDC queries

   CREATE CHANGEFEED WITH key_column='b', unordered AS SELECT cdc_is_delete() FROM foo

5. selecting on crdb_internal_mvcc_timestamp works

   CREATE CHANGEFEED WITH key_column='crdb_internal_mvcc_timestamp', unordered AS SELECT crdb_internal_mvcc_timestamp FROM foo WHERE crdb_internal_mvcc_timestamp > 0

6. using CDC queries with event_op works

   CREATE CHANGEFEED WITH key_column='b', unordered AS SELECT * FROM foo WHERE event_op() = 'delete'

Jira issue: CRDB-33949

[blathers-crl[bot]]

cc @cockroachdb/cdc