Cockroach: sql: support ALTER TABLE ALTER COLUMN SET NOT NULL

[knz]

This is distinct from #9851:

CockroachDB should be able to add the NOT NULL specifier on a column with ALTER, using a job that asserts that indeed no values are NULL.

[tomholub]

Ability to do this (w/o creating and moving data to a new column) would be appreciated!

[sakalaramesh]

We would also appreciate having this feature as we also run into this issue and ended up running without this constraint

[knz]

@awoods187 ^^

[lopezator]

Just came here to say +1 😄

[jordanlewis]

cc @dt moving to schema change's project board.

[joshimhoff]

+1

[awoods187]

@joshimhoff how do you feel about a temporary solution of create table as?

[dt]

In progress: https://github.com/cockroachdb/cockroach/pull/37554

[joshimhoff]

@awoods187 LGTM, thx for pointer.