Add forwarded message entity

Quoting the [discussion](https://matrix.to/#/!QtykxKocfZaZOUrTwp:matrix.org/$15359952391039791ucBgy:matrix.org) on Matrix HQ (and adding some details)

I mean forwards similar to telegram, which you get additional metadata i.e. the original author, the channel that message came from, original message id (so clients could implement jumping to that context like replies) etc.

> the reason it hasn't been done is you can't verify if the message is legit unless you're in the other room. cross-room replies aren't a thing for the same reason

Aren't these events signed?

> Not in a way clients can see currently

> that would be a very server-side thing, but with that it might be doable

> it would probably require an additional endpoint just to forward messages and support to properly check the signature on receiving servers

Start by reading the linked Matrix HQ discussion and the additional details in this issue. Identify the required forwarded-message metadata, how authenticity and signatures would be checked across rooms, and whether an additional forwarding endpoint is needed. Done means the specification has a decided, implementable design for the entity and its verification flow.

Written by the indexing model from the issue text.