Tempel: add MFA support

[ptaoussanis]

Task: Make it easier for users to add MFA/HOTP/TOTP/etc. to their Tempel-protected data.

Motivation: Using MFA with Tempel is already possible, but it might not be obvious how to do it. I could either document it, or just build it in as a direct feature. Having it as a direct feature might be nice, and shouldn't be too much work.