opt: add support for aggregate FILTER

[jseldess]

PR: cockroachdb/cockroach#34077

From release notes:

FILTER expressions are now supported by the cost-based optimizer. [#34077][#34077] {% comment %}doc{% endcomment %}

[jseldess]

@rmloveland, does the unsupported statement error here need to be changed given the above? https://www.cockroachlabs.com/docs/v2.2/cost-based-optimizer.html#view-query-plan

[rmloveland]

@rmloveland, does the unsupported statement error here need to be changed given the above? https://www.cockroachlabs.com/docs/v2.2/cost-based-optimizer.html#view-query-plan

Yes. I considered doing a small PR right now, but I have issues #4366 and friends which mean that section will need a full rewrite anyway due to all of the various EXPLAIN changes. The smaller PR now would be wasted work (and may also lead to merge conflicts since this page is beginning to have many things on it ^_^ )

The docs that state we support the feature from this issue were added in commit 7fbce7b so I'll close this one and let the erroneous text be washed away by those other impending work items.