Custom fields for common networks

### Problem

Im not sure how much the standard allows... But a part of why i don't use Contact apps is that i cannot add "non traditional" contact informations, such as:
- Discord handle
- Bluesky handle (/mastodon)
- Personal website
- Toyhouse handle
- Github handle...

Which bums me out because those are also kind of legitimate if i want to reach out to someone

### Proposal

Allow adding custom fields, with some common ones present like the above mentioned

I am not sure how allowed this is by the service provider in the backend however. It would be pointless if these are filled by the user but then not synchronized by online-accounts.

### Prior Art (Optional)

_No response_