sql: panic: add column T[] unique

[madelynnblue]

create table t(); alter table t ADD COLUMN c int[] UNIQUE;

panic while executing 1 statements: ALTER TABLE _ ADD COLUMN _ INT[] UNIQUE; caused by unimplemented: column c is of type ARRAY and thus is not indexable

Found with RSG.

[madelynnblue]

This panics 2.0.2 so we should fix that in master and backport.

[solongordon]

Sentry issue: COCKROACHDB-GY

[bobvawter]

checkColumnsValidForIndex depends on the new column being present in the TableDescriptor before being able to perform the check. This means that an AddColumnMutation followed by an AddIndexMutation winds up actually not performing the index preflight check since the column (with un-indexable type) isn't yet present.

@vivekmenezes I'll update the ADD COLUMN ... UNIQUE flow to ensure the pre-flight checks are run and then fix this more generally in the work you and I have discussed to make TableDescriptor changes operate as a queue of functions.

[knz]

@bobvawter note there are multiple types besides arrays that are non-indexable. for example JSON. Perhaps make multiple tests?