JOIN support improvement

Is your feature request related to a problem? Please describe.

There is limited support for JOIN queries which needs improvement. In particular, post processing such as filtering and aggregation after inner/outer join is most wanted. This capability is required by PPL lookup command.

Here are the feature requests from community:

1. https://github.com/opendistro-for-elasticsearch/sql/issues/505: Left join
2. https://github.com/opendistro-for-elasticsearch/sql/issues/351: Multi-join
3. https://github.com/opendistro-for-elasticsearch/sql/issues/301: Aggregate after join
4. https://github.com/opendistro-for-elasticsearch/sql/issues/289: Select function after join
5. https://github.com/opendistro-for-elasticsearch/sql/issues/236: Aggregate after join
6. https://github.com/opendistro-for-elasticsearch/sql/issues/221: Comma join
7. https://github.com/opendistro-for-elasticsearch/sql/issues/124: Left join with WHERE
8. https://github.com/opendistro-for-elasticsearch/sql/issues/110: Aggregate after join
9. https://github.com/opendistro-for-elasticsearch/sql/issues/987: Multi-join

Describe the solution you'd like
N/A

Describe alternatives you've considered
N/A

Additional context
Add any other context or screenshots about the feature request here.

Start by reviewing the nine linked JOIN requests, especially the items covering left joins, multi-joins, aggregation, and post-join filtering. The issue names no files, tests, entry points, or defined completion condition, so the scope and acceptance criteria must be clarified before implementation.