[Standards] Extended contacts

The current contact implementation is lacking, which causes having lots of fragmentation and duplicates in contacts. Here are some scenarios: * XEP proposal1: meta contacts. A single person John Doe has multiple accounts doe1(a)ex1.com, doe2(a)ex2.com, doe3(a)ex3.com ... It would make more sense to have an "extended contact" or a "meta contact" that combines all of these addresses. * XEP proposal2: once meta contact is implemented, it would be desirable to have a field for a personal note. For example, this is available in Slack and Discord. In 2020 there was an XMPP sprint for "studying Discord and bringing its interesting features" (https://wiki.xmpp.org/web/Sprints/2020_November_Online), and I think this is an interesting and useful feature. * XEP proposal3: better contact integration. The contacts data on XMPP apps of Android is very isolated from the rest of apps. All other messaging apps interact well with the device contacts and phone numbers, except for XMPP because it doesn't use phone numbers as identifiers. But once we have XEP proposal2 and we can add personal notes, we can also add a field for phone numbers for contacts that would make it more interoperable with device contacts that are identified with phone numbers. For these 3 proposals, I was moving point by point to explain the motive behind the next proposal which actually combines all of the three previous points. * XEP proposal4: vCard compatible contacts. The vCard standards solves all three points since it 1) accepts multiple phone numbers and emails, 2) Include note, birthday, company and other miscellaneous fields, and 3) are supported natively by phone contacts. As of vCard v4, it has XML property which "is used if the vCard was encoded in XML (xCard standard) and the XML document contained elements which are not part of the xCard standard." * XEP proposal5: once vCard standard is supported, it would be easy to have XMPP as a WebDav server. Android app like Conversation have access to all contact information and can easily sync contact information with phone contacts. * XEP proposal6: It remains to clarify how XMPP clients deal with contacts that have no XMPP handle. Signal chose the position of not showing them in app at all. WhatApp prepares an SMS to invite them. This might be left to the client, but as a Slidge gateway user, it would be convenient to check phone numbers as a username handle for the current server. E.g: if a phone number is +18884440000, client would check the main gateway servers setup by users +18884440000(a)example.org, +18884440000(a)whatsapp.example.org, and +18884440000(a)telegram.example.org. If non of these exist, then the client can proceed with inviting them to XMPP.. As for the security, a simple disclaimer like: "XMPP found these gateway users but couldn't verify their ownership" should suffice. * XEP proposal7: contact-related features might need revision like XEP-0140 (contact groups), as this is implemented in vCard too.