Enable parents.children.create(...) syntax

[scriptogre]

Description

Add create() method to ReverseRelation for ForeignKeyField.

Provide an ergonomic way to create related objects, and better fulfill expectations of people coming from Django's ORM.

Before:

event = await Event.create(tournament=tournament, ...)

After:

event = await tournament.events.create(...)  # Now possible

The implementation is simple (just ~5 lines of code):

1. Check if parent instance is saved in db (otherwise raise OperationalError)
2. Automatically inject relationship into kwargs
3. Call remote model's create method (with the updated kwargs)

Motivation and Context

1. Django compatibility: Developers migrating from Django probably expect parent.children.create() to work.
2. Consistency: Tortoise already implements such methods for ManyToMany fields (.add(), .clear(), .remove()). ForeignKey should have the same level of support.
3. Developer Experience: Eliminates the need to manually specify the foreign key relationship, reducing boilerplate.

How Has This Been Tested?

Added tests for successful creation and error handling with unsaved instances.

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]

CodSpeed Performance Report

Merging #1991 will not alter performance

_{Comparing scriptogre:enable-create-via-reverse-relations (4c495a5) with develop (c4f601e)}

Summary

✅ 16 untouched benchmarks

[henadzit]

I just merged a fix for CI. Can you please rebase your branch?

[coveralls]

Pull Request Test Coverage Report for Build 17092327155

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on enable-create-via-reverse-relations at 89.637%

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Totals
Change from base Build 17090868824:	89.6%
Covered Lines:	6676
Relevant Lines:	7266

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💛 - Coveralls

[henadzit]

LGTM