Support alter cdc backfill rate limit

[kwannoel]

Currently alter table t1 set backfill_rate_limit = xx; is not supported for cdc backfill tables.
They should support alter rate lmit as well for table.

Further we should have some tests for this case.

[StrikeW]

I think the syntax for cdc table should be consistent to #17911, that is

alter table t1 set backfill_rate_limit = xx;

right?

[kwannoel]

I think the syntax for cdc table should be consistent to #17911, that is

alter table t1 set backfill_rate_limit = xx;

right?

Yeah exactly. Let me amend the description.

[StrikeW]

Done in #17989