Create an RFC to make capabilities negotiation extensible

[mcy]

There are a few opens, namely:

• Do we want to include the "PFM Support" et al bits? They're underspecified right now and it might make more sense to add them in a future RFC.
• Is being able to send 16k byte capabilities useful? I think it's not but I included it for completeness. We can instead switch to something like the Zigzag encoding and use only short integers.