[feature request] ability to search for messages in a room

### Your use case

#### What would you like to do?
I would like to be able to search for messages in a room. Even better search for messages across joined rooms/spaces but let's start small and focus on relative priority, so search per room first.

#### Why would you like to do it?
Finding previous messages is often very important and can't be done right now (well, just manually)

#### How would you like to achieve it?
Like in element-web/desktop, which would also help improving UI consistency across platforms

### Have you considered any alternatives?

Scrolling manually through the timeline to look for an old message is a huge PITA, for obvious reasons. I'm using threads as a workaround (poor-man's bookmark) for messages I already know I'll refer back to later. Obviously none of these can or should be an alternative to an actual message search.

### Additional context

I couldn't find any related issue for ElementX (Android and iOS) but consider the lack of a message search to be a major one. Thus any solution found here should become part of the common/shared portion of ElementX, i.e. also become part of the iOS variant.

### Are you willing to provide a PR?

No

No files or tests are named. Start by reviewing message-search behavior in element-web/desktop and the shared ElementX/iOS architecture, then define the per-room scope and how it should fit both clients. Done means users can search messages within a room with behavior and UI consistent across platforms.

Written by the indexing model from the issue text.