support custom error codes in options check #19953

Feature
mypy options check should recognize custom error codes defined in plugins

Pitch
We have a number of custom mypy checks and would like them to have distinct error codes, allowing them to be reported / ignored / enabled / disabled just as built in errors.

mypy appears to mostly support custom error codes defined by a plugin, since the ErrorCode constructor registers with the global collection of errors, and everything operates as expected (reporting, using type: ignore[], and disabling... but see below).

The problem comes up when referring to custom errors codes in options, triggering Invalid error code(s):

• on command line with --disable-error-code, etc.: fails outright with non-zero code
• in pyproject.toml disable_error_code etc.: outputs a warning but does not cause non-zero exit, and the reference is properly honored

I believe this can be rectified by deferring the error code check until after plugins are loaded, but realize that may be tricky to implement.