colexec: fully support distinct spec

[jseldess]

Jesse Seldess (jseldess) commented:

PR: cockroachdb/cockroach#61582

From release notes:

Previously, in some special cases (UPSERTs, as documented in this issue), the support of the distinct operations was missing in the [vectorized execution engine](../v21.2/vectorized-execution.html]. This has been added, and such operations will be able to spill to disk if necessary. However, in case the distinct operator does, in fact, spill to disk, there is a slight complication. The order in which rows are inserted can be non-deterministic: for example, for a query such as INSERT INTO t VALUES (1, 1], (1, 2), (1, 3) ON CONFLICT DO NOTHING, with t having the schema a INT PRIMARY KEY, b INT, it is possible that any of the three rows are actually inserted. PostgreSQL appears to have the same behavior. [#61582)[#61582) {% comment %}doc{% endcomment %}

Jira Issue: DOC-1394