additional, custom, local info in Peer

Hi,

although I try to minimize states, in an upcoming project I could use additional, local state information inside a `Peer`, e.g.

- last "heartbeat" timestamp
- a "busy" state
- additional ready/in-sync state on top of `isConnected`

Normally I would subclass `Peer` but MultiPeerKit would not use it obviously. In rare cases I would use objc dynamic runtime and add an "object association" but `Peer` is a struct.

Currently, a "wrapper" would be needed to map an additional local struct to a peer. This creates update problems, keeping the local struct and the peer list in sync - an ugly and weak approach.

I thought about the common `userInfo` dict approach each `Peer` could offer for custom key-values. Sadly this is not very Swift-friendly although a local extension with computed properties could hide the dict.

This `userInfo` dict is probably the least invasive approach for the current code, or do you have another idea in mind that is better suited?