Introduce dynamic table append

[kgyrtkirk]

Dynamic table appaned TABLE(APPEND(...))

This is essentially an SQL level syntactic sugar which matches columns by name from multiple tables.
Suppose you have 3 tables:

• table1 has column1
• table2 has column2
• table3 has column1, column2, column3

It is possible to create an union view of the above tables is possible by using UNION ALL:

SELECT column1,NULL AS column2,NULL AS column3 FROM table1
UNION ALL
SELECT NULL AS column1,column2,NULL AS column3 FROM table2
UNION ALL
SELECT column1,column2,column3 FROM table3

However depending on the size of the table's schema it might be quite complicated to do that; TABLE(APPEND('table1','table2','table3')) represents the same in a more compact form.

It adds one additional thing

• if a column may have conflicting types (same column defined with different types in the inputs) - ColumnType.leastRestrictive is used to get a common type for them

Release note

Introduce dynamic table append - SELECT * FROM TABLE(APPEND('table1','table2')) implicitly creates an union based on the table schemas.

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[rohangarg]

Thanks so much for the changes @kgyrtkirk :)
I have left a few comments on the changes, please let me know your thoughts

[vtlim]

The new docs look great. I left some suggestions to improve scanning and readability.

[vtlim]

• Since this has the same native query syntax as union should this content go within the existing union section?
• Suggest removing the SQL syntax from the header title, so just have #### Dynamic table append

[kgyrtkirk]

I've moved the section to be the next after union

what do you think:

• should this be made a subsection of union?
• should I remove the native as that matches the standard union's behaviour?

[vtlim]

I like both of those ideas. Since the native union behavior is the same, the new content can go in there, and removing the repeated native section won't have people wondering what the difference is.

[vtlim]

One comment on heading styling. With that and the changes in #15897 (comment), docs should be all set.

[vtlim]

LGTM for docs

[rohangarg]

LGTM 🚀