Support override for `attr-defined` errors (something like a promise to mypy that the attr is defined going forward)

Feature

There are cases where mypy spits out an attr-defined error when accessing bar.foo; these can be ignore per-use with a # type: ignore[attr-defined], however, there doesn't currently seem to be a way to tell mypy that bar does indeed define foo.

In this case, it'd be nice if there was a way to tell mypy that bar does define foo, so that bar.foo can be used without spamming # type: ignore[attr-defined] comments everywhere.

How this is done I don't have an opinion about, but for example, it could be something like: # type: bar.foo: int.

Pitch

There are multiple useful cases of this:

1. Although this is not the primary motivation, it would cover the case brought up here: https://github.com/python/mypy/issues/15085

2. This could allow type hinting for attributes added after a class definition. One example would be for users who enjoy using a TRACE log level with the logging module. That is, if a user adds a TRACE logging level to the logging module such as shown here: https://stackoverflow.com/questions/2183233/how-to-add-a-custom-loglevel-to-pythons-logging-facility
   In this instance, this would allow use of logging.trace and logging.TRACE without requiring #type: ignore[attr-defined] used at every occurrence.
   In general, this handles the case where you have something like:

class A:
    pass
A.B = "foo"    # Some comment to promise mypy that A has an attr named B defined

3. Additionally, this can allow users to better handle when 3rd party libraries do not play nice with mypy. For example:

from cryptography.hazmat.primitives.serialization import load_ssh_private_key

This gives an attr-defined warning, although this import statement works just fine.

Beginne damit nachzuverfolgen, wie mypy attr-defined-Diagnosen meldet und Kommentare vom Typ type: ignore[attr-defined] pro Verwendung behandelt. Definiere die vorgeschlagene Promise-Syntax und ihren Geltungsbereich anhand der Beispiele im Issue und füge anschließend Tests hinzu, die zeigen, dass versprochene Attribute keine wiederholten Fehler mehr erzeugen, während weiterhin Fehler für andere fehlende Attribute erzeugt werden.