Add max length to secure_filename()

[vytas7]

There is already a TODO in the source code (falcon/util/misc.py):

    # TODO(vytas): max_length (int): Maximum length of the returned
    #     filename. Should the returned filename exceed this restriction, it is
    #     truncated while attempting to preserve the extension.

Actually implement this max_length parameter according to the above note.

Also add a new option to MultipartParseOptions that is used when evaluating part.secure_filename.

In order to avoid a breaking change, both values should be None by default, with a note that starting from Falcon 5.0, this value will non-nullable, with the new default of NN characters (please suggest).

[Krishn1412]

Hey @vytas7 , can I pick this up?

[vytas7]

Sure, go ahead!

[Krishn1412]

Can you take a look at this? #2421