Support pronouns for contacts

Is your feature request related to a problem? Please describe.

See https://github.com/nextcloud/server/issues/40017. For users, we can already set pronouns. But for (external) contacts, I didn't find a way.

Describe the solution you'd like

https://github.com/nextcloud/contacts/issues/4342#issuecomment-2676266739 mentioned modern alternatives to RFC6350 that support this.
So I guess implementing one of these would be a good way to resolve my issue.

Describe alternatives you've considered

Yes, there is a 'gender' field but sometimes I know the pronouns a contact prefers, but not what gender they identify as (e.g. does 'she/them' imply 'female'?). Just setting those to 'other' gender might loose that information. I can add it in the notes, but when writing a batch-mail to contacts, I would like to be able to programatically pick the corresponding prefered pronouns.

Additional context

Note that neither I want to start a debate about gender and pronouns nor do I want to engange in gender studies here. I just consider it polite to address contacts the way they prefer and would like to have that information in my address book in a unified way.

Start by reading the linked Nextcloud server and Contacts discussions, then inspect how external contacts and vCards based on RFC6350 are represented and exchanged. Identify a supported modern alternative for storing pronouns and define how the value should be available for programmatic batch-mail selection.