FEATURE REQUEST: Import encrypted seed files for ColdCard, Passport, Etc.

Currently both ColdCard and Passport backup their seeds to SD cards, and encrypt them using 7zip _(corrected)_.

I think (but don't know) that they are both just storing the seed words as a list.

However, they both have different schemes for how they do the gzip password.

Part one of this issue is to document what these two hardware signers do, what the resulting decrypted text look like, how and rationale for how the password is chosen (and how they suggest the password be stored) and if any other hardware signers backup to gzip files on SD cards or other media as well. We should share these details in the airgapped repo and a new thread in discussions there. Thus the first assignee for this issue to @shannona.

Once we've documented this, we should have a call out to the airgap community for a single standard for how these backups SHOULD be done. (I know that Blockchain Commons would prefer the community ur:crypto-envelope, but that is likely too far down the roadmap as it is new crypto and requires security reviews.)

If two or more companies want to support the same gzip'ed seed method (an existing one or a new one) we should support reading and writing these encrypted seeds from the file system in Gordian Seed Tool as a reference implemention. @shannona should assign this back to me at that point to puzzle when that feature should be added as a milestone issue for delivery.

cc/ @FoundationKen @zachherbert