Sieve: Show installed sieve extensions

[kesselb]

Feature Request

As end user I would like to know which sieve extensions are installed.

A method to retrieve the extensions is provided by ManageSieve: https://github.com/horde/ManageSieve/blob/c3abba972bee22731e63a6af737e77704aef838c/lib/Horde/ManageSieve.php#L505-L515

[ChristophWurst]

FYI for good first issue we should have clear instructions about what is required to do the change. People who are totally new to Nextcloud (Mail) should be able to get a sense of where changes are to be applied.