R1:a69db10c5e15

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]…