Impose FilterableValue in UniqueColumn::find

[Centril]

Description of Changes

Fixes #2272.

This PR does the following:

1. Imposes that the sought value column value for UniqueIndex::find must be FilterableValue.
2. Makes FilterableValue a sealed trait so that we may intentionally grow the set and make it un-sealed at a time of our choosing.
3. Adds #[diagnostic::on_unimplemented(...)] to FilterableValue for better diagnostics.
4. Adds UI tests to ensure that .filter(..) and .find(...) impose FilterableValue.

The PR does not impose the constraint on table columns themselves.
Perhaps that is what we want to do, as it would allow us to also impose the check on the semantic schema level in the host.
This would be more foolproof.

API and ABI breaking changes

Yes but its a bug fix.

Expected complexity level and risk

1

Testing

A UI test has been added.

[coolreader18]

I'm not sure if this is what was intended by the ticket, but if it is, this LGTM.

[bfops]

(Which ticket is this tied to?)

[cloutiertyler]

I'm not sure if this is what was intended by the ticket, but if it is, this LGTM.

I think you need to be sure in order to approve the PR.

[coolreader18]

FTR: Mazdak said that @cloutiertyler said that this PR implements the intended behavior.

maybe it's not the right thing

[coolreader18]

@gefjon this seems to be your ticket - could you review this PR to confirm if it's what you intended by #2272?

[gefjon]

Love the test!