Use hash joins when nested loop joins are not feasible

[systay]

Description

The join we to use in vtgate is the apply join, also known as nested loop join. It's algorithm is pretty straight forward - we execute the query on the LHS of the join, and then, for each row in this result, we extract the join columns from the left and push them down on the RHS of the join, and use them for filtering there.

This works well in most situations. One issue is that we can't have the outer side on the LHS of the join - we won't see the values that are coming from the RHS that don't match anything on the left.

Another limitation is that we can't really have LIMIT on the RHS either. Our engine primitives understand a call to the Execute method as a new query. Since we will need to do multiple Execute calls on the RHS, LIMITs there will produce too many rows. A LIMIT 10 will produce 10 rows per row on the LHS, and that is not correct.

To solve this, we have a couple of options. One would be to introduce a query scope for the engine primitives.
In this PR I'm trying using the hash join engine primitive for these queries. Since it executes the LHS and the RHS on their own and then joins the result, we can allow outer joins and limits on either/both sides.

Related Issue(s)

Fixes #11879

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[systay]

The produced plans still need optimising - this PR only makes the queries possible, they are still going to be very slow. Optimisation work incoming

[vmg]

Looking gucci my friend