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.

Inizia tracciando il modo in cui mypy segnala le diagnosi attr-defined e gestisce i commenti type: ignore[attr-defined] per singolo utilizzo. Definisci la sintassi di promise proposta e il suo ambito usando gli esempi nell’issue, quindi aggiungi test che mostrino che gli attributi promessi non producono più errori ripetuti, mentre gli attributi mancanti non correlati continuano a produrli.