Provide a devtool for manually verifying other devices

[andybalaam]

Fixes element-hq/element-meta#2751

This provides a dialog inside devtools:

(You can open devtools by typing /devtools in the composer in any room.)

This tool cross-signs the device with the supplied device ID, making it verified, so it can send and receive messages to/from other users without red shields appearing on its messages, and especially, so it can send/receive to/from users who have "Exclude Insecure Devices" turned on.

For symmetry, we also provide the required information in case we want to do this in another client:

We also provide a slash command:

/verify <device-id> <device-fingerprint>

This does the same thing as the dialog.

Here it is in action:

rec-2025-06-06_13.05.33.mp4

This code is partly reconstructed from #28588 where a similar feature was removed.