feat: add DISTINCT ON support for PostgreSQL.

[VladislavYar]

Description

Added DISTINCT ON (fields) support for PostgreSQL to QuerySet.distinct(*fields).

Previously, .distinct() only supported plain DISTINCT (no arguments). Now, passing field
names generates DISTINCT ON (fields) on PostgreSQL, keeping one row per unique combination
of the specified fields. The feature is fully propagated to .values(), .values_list(),
and .only().

Also added:

• skipCapability decorator to tortoise.contrib.test — the inverse of requireCapability,
  skips a test when the specified capabilities match.
• _apply_db() helper in _ChooseDBMixin that consistently sets the DB connection and
  switches the query builder to PostgreSQLQueryBuilder across all query classes.

Motivation and Context

PostgreSQL's DISTINCT ON is a powerful feature that standard DISTINCT cannot replace —
it allows selecting one representative row per group without a GROUP BY, while retaining
full model objects. This is commonly needed for "latest per group" or "first per category"
queries.

How Has This Been Tested?

Added tests/test_distinct.py covering:

• Basic DISTINCT ON with and without ORDER BY
• Multiple DISTINCT ON fields
• Combined with .values_list() — single field, multiple fields, field outside DISTINCT ON
• Combined with .values() — same variations
• Combined with .only()
• ORDER BY respects the selected row within each group
• OperationalError when ORDER BY doesn't start with DISTINCT ON fields
• OperationalError when used on a non-PostgreSQL database

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 VladislavYar:distinct-by-specific-fields (fa341d0) with develop (403a4dc)}

[waketzheng]

So many lines of this function. I would prefer:

def skip_dialect(*names: str, connection_name: str = 'models') -> Callable[[_FT], _FT]:
    return requireCapability(connection_name, dialect=NotIn(*names))

[waketzheng]

How about @test.requireCapability(dialect=NotIn("postgres"))

[VladislavYar]

You're right. I removed the skipCapability function and changed it to requireCapability(dialect=NotIn("postgres")).