Fix "Creation of dynamic property is deprecated" PHP 8.2 error in MimeMailParser

Summary:
Add #[AllowDynamicProperties] attribute to avoid the deprecation notice.

See https://php.watch/versions/8.2/dynamic-properties-deprecated

# ERROR 8192: Creation of dynamic property MimeMailParser::$parts is deprecated at [/var/www/html/phorge/phorge/externals/mimemailparser/MimeMailParser.class.php:137]
#  #0 MimeMailParser::parse() called at [<phorge>/externals/mimemailparser/MimeMailParser.class.php:126]

Closes T15765

Test Plan: See T15765

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15765

Differential Revision: https://we.phorge.it/D25563