Allow the user to bind a encrypted Stratis pool to a Tang server.

This should reuse the designs and code from the LUKS part of Cockpit.

Initial thoughts:

• Adding a tang server should show the advertisement and ask the user to confirm the fingerprint, as we do with LUKS.

• The user should be able to remove the tang server again from a pool, with the same warnings/protections with do for LUKS. (Ask for passphrase to confirm that it is still known, allow removal anyway with stark warning.)

• Starting (unlocking) a pool and adding/removing devices should automatically try to use the tang server and only prompt for the passphrase if that fails.