If a public room supports peeking, allow searching

[Miepee]

Is your feature request related to a problem? Please describe.
Sometimes I want to search for specific stuff in a room when peeking, without having to join it. It is already possible via peeking to get the whole room's history, so searching in there would make it easier to only get specific information.

Describe the solution you'd like
Allow searching in public rooms that support peeking.

Additional context
Was originally brought up in this issue: matrix-org/matrix-static#111
EDIT: wasn't sure whether to open the issue here, or in the matrix-spec repo.

[uhoreg]

EDIT: wasn't sure whether to open the issue here, or in the matrix-spec repo.

matrix-spec is the correct place; I've transferred the issue.

Some notes: the /search endpoint endpoint by default searches all rooms that a user is in or has been in, but you can specify a filter to limit it to certain rooms. If we do allow searching in peekable rooms, we probably don't want to make the search include all peekable rooms by default, as that could be very large. But we could make it so that if the user specifies a room (or rooms) in the filter, and the room(s) is/are peekable, then it can be searched.