Description

This PR:

• Adds support for referencing relationship fields in F(), e.g. F("author__name") that automatically introduces joins to the query
• Allows referencing annotations in F(), e.g. IntFields.annotate(intnum_plus_1=F("intnum") + 1).annotate(intnum_plus_2=F("intnum_plus_1") + 1)

In order to achieve that, the following implementation details changed

• The behavior of Expression.resolve was unified to always return a ResolveResult. Previously it could have been either a tuple or dictionary.
• F was changed to be an Expression instead of pypika's Field. This allowed to move the F's resolution code inside F.resolve and simplify the resolution code in general because F can be treated as other Expressions.
• CombinedExpression was introduced to handle arithmetic operations on F(), e.g. F("a") + 1. It was inspired by Django's CombinedExpression and replaces pypika's arithmetic. CombinedExpression is a child of Expression, so it can be resolved without knowing the exact type of the object as opposed to pypika's ArithmeticExpression that was used previosly.

As part of this PR, I had to change how custom functions can be defined. It changed from

from tortoise.expressions import F
from pypika.terms import Function

class JsonSet(Function):
    def __init__(self, field: F, expression: str, value: Any):
        super().__init__("JSON_SET", field, expression, value)

to

from tortoise.functions import Function
from pypika.terms import Function as PupikaFunction

# it is a tortoise Function now, not Pypika's Function
class PypikaJsonSet(Function):
    def __init__(self, field: F, expression: str, value: Any):
        super().__init__("JSON_SET", field, expression, value)
    
    database_func = PypikaJsonSet

Effectively now custom functions are supposed to be a tortoise Function, not a Pypika's Function. The change had to be done because the Pypika's Function is no longer compatible with F and, frankly, having the interface that accepts both Pypika's functions and tortoise functions is a bit messy and makes the code quite convoluted.

Motivation and Context

This introduces functionality supported by Django but also there is a clear demand for these features in the tortoise community. The following issues should be fixed by this PR

• fix 🐛 Issue: Model.annotate(...).update(...) not working #1703
• fix Multiple annotations don't work #683
• fix Use F expression in Annotation #995
• fix Annotate by field of the related model #477
• fix Support annotate F() expression cross model relation #1496

How Has This Been Tested?

• Tested on a sample web app
• Added tests

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.