Rename attribute `output_type` on the Query Expression API

[jorgecarleitao]

Since the output_type used in the Query Expression API is becoming part of the Public API in the 1.7,
I would like to propose to rename it before 1.7 is out. Here is why:

output_type is a property of classes that follow the Query Expression API and is defined on any object that follows the Query Expression API as

@property
def output_type(self):
    return IntegerField()  # example, it only requires to be a subclass of `Field`

However,

print(type(IntegerField()))  # <class '__main__.IntegerField'>
print(type(IntegerField))    # <class 'type'>

IMHO, output_type can be misleadingly interpreted as being a type rather than an instance of (a subclass of) a Field.
I imagine it would be very easy to someone write

@property
def output_type(self):
    return CustomField

instead of

@property
def output_type(self):
    return CustomField()

Given that some types that comply with the Query Expression API already have the attribute source (Aggregate, Col, and GeoAggregate), one possibility would be use just output.

[Tim Graham]

Bumping to release blocker so we make a decision before 1.7 is released.

[Andrew Godwin]

Could we not enforce a check that it is a field instance and not a class?

That said, if the impact of renaming it is minimal, let's do it now.

[jarshwah]

If it were to be renamed, I vote for "output_field", which gives a little more information about what it actually is - an instance of a model_field. Documentation can describe what the current name is, but no such documentation exists (yet) that is clear.

[Anssi Kääriäinen]

The current name is something I just picked while hacking - there wasn't that much though put into it. The reasoning is something along lines "what data type the DB produces for the expression", hence output_type. Fields happen to represent also database data types, so that is why field instances are used.

If some better name can be found I don't have any objections against changing the name.

[Simon Charette]

Now we just have to agree on a name. @jorgecarleitao what do you think of output_field?

[jorgecarleitao]

I agree output_field, even more than output :)

[Tim Graham <timograham@…>]

In 95cc0e15b45dd0986bdfd9fa3fedee4550c744c8:

Fixed #22819 -- Renamed output_type -> output_field in query expression API.

Thanks jorgecarleitao for the suggestion.

[Tim Graham <timograham@…>]

In aa10f57d9460f121dea2ec2635e478ed02cc18b5:

[1.7.x] Fixed #22819 -- Renamed output_type -> output_field in query expression API.

Thanks jorgecarleitao for the suggestion.

Backport of 95cc0e15b4 from master