Add union query support

[seladb]

Add union query support in an API similar to Django's API

Addresses these issues:
#1507
#2056 (partially)

Description

Add union query support:

qs1 = Tournament.filter(name="T1").only("id")
qs2 = Reporter.filter(name="R1").only("id")

result = await qs1.union(qs2)

As opposed to Django, this API supports result of multiple models (in Django all results will be instances of the first model).
However, similar to Django, the requested field names and types should be the same across all queried models. This is because of how DBs support union queries.

Motivation and Context

It addresses the following issues:
#1507
#2056 (partially)

Also - it brings more feature parity with Django.

How Has This Been Tested?

Added test coverage for all written code. It was tested with MySQL, Postgres and SQLite.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing seladb:add-union (ba5973e) with develop (e6c7f64)}

[seladb]

MSSql limit syntax is different but it's not currently supported in _SetOperation which has its own implementation of _limit_sql() instead of using the dialect SQL. I guess we can leave it as a TODO for later

https://github.com/tortoise/pypika-tortoise/blob/378f6145f7529d88fa58510851d80454b742406e/pypika_tortoise/queries.py#L672