Add more metadata to m.room.member events (for bots and ASes) (SPEC-113)

Nobody has claimed this yet.

Lifted from AS API discussion and other linked issues:

Sometimes we want to identify bridged users (e.g. nick completion as per original issue). There is no way to reliably do this currently: you have to do `user_id` regex parsing.

We need to spec out where we can find this out (extra JSON keys on `m.room.member`, something in the profile, etc) and what this should look like (URIs?) so informed clients can make decisions for things like visual representations, auto-complete, etc.

Original issue:
bq. If a Matrix user tab-completes to bing someone on IRC, it will autocomplete as (IRC) Arathorn or @​irc_Arathorn:matrix.org. This looks horrid to people on the IRC side of the bridge; we should either support autocompleting their IRC name on Matrix (probably the best bet) or the bridge could autoconvert through to the correct local idiom on the IRC side of the bridge. Given how ugly @​irc_Arathorn:matrix.org is in a message, and that bingers are for humans rather than machines, I'd prefer that we made the clients smarter about how they autocomplete, and/or expose IRC nicknames more intelligently to Matrix.

```
.. TODO
Sometimes application services need to create rooms (e.g. when lazy loading
from room aliases). Created rooms need to have a user that created them, so
federation works (as it relies on an entry existing in m.room.member). We
should be able to add metadata to m.room.member to state that this user is an
application service, a virtual user, etc.
```

(Imported from https://matrix.org/jira/browse/SPEC-113)

(Reported by @Kegsay)

Start with the m.room.member event requirements and the AS API discussion described in this issue, including the original IRC bridge use case. Compare the possible metadata locations and representations mentioned in the issue. Done means the specification clearly defines where this information is exposed and how informed clients can use it.

Written by the indexing model from the issue text.