[Story] Add a caption when sending a media file

[pixlwave]

Description

• As a user
• I want to be able to include a caption when sending some media
• So that I can add context to the media in the same message.

Acceptance criteria

• A composer is shown at the bottom of the media preview screen.
• Its caption is optional: when nothing is entered, no caption will be added to the media.
• Entering formatting using markdown should work as it does in the regular composer.

Size estimate

None

Dependencies

• None

Out of scope

• As EX doesn't (yet) support sending multiple pieces of media, we don't need to consider what would happen in this instance.
• We don't need to use the RTE to create formatted captions.

Open questions

### Questions

Subtasks

### Android
- [ ] https://github.com/element-hq/element-x-android/issues/2573
- [ ] https://github.com/element-hq/element-x-android/issues/3884

### iOS
- [ ] https://github.com/element-hq/element-x-ios/issues/3508

### Rust

### Other