Ability to verify a single device using SAS or session key fingerprint

### Your use case

#### What would you like to do?
I'm using [matrix-nio](https://github.com/poljar/matrix-nio) with e2ee, which is the base for several python SDKs/bot frameworks. It supports fingerprint verification (i.e. it can print its fp to check on another device) and to-device-SAS with emoji and decimal modes.

#### Why would you like to do it?
It's possible to verify using the above methods in element web. I want to do the same on mobile.

#### How would you like to achieve it?
Viewing a user's profile, there is the security section with a button "Verify".
Clicking the button pops up a modal with the single choice "start verify" which starts an in-DM verification.
Add the ability to view devices here (such a view exists and is accessible after verification) and click a single device to send to-device verification requests.

### Have you considered any alternatives?

The spec states
> Verifying between two different users should be performed using in-room messages, whereas verifying two devices belonging to the same user should be performed using to-device messages.

and arguably nio should be updated to support in-room verification. However the verb used is "should" and not "must", i.e. to-device verification should still be possible between users.

### Additional context

_No response_

### Are you willing to provide a PR?

No