Add support for verifying apps using the F-Droid Binary Transparency Log

[UntrimmedParasail]

https://gitlab.com/fdroid/f-droid.org-transparency-log

This is a log of the signed app indexes downloaded from https://f-droid.org/repo/ This is stored in a git repo, which serves as an imperfect append-only storage mechanism. People can then check that any file that they received from that F-Droid repository was a publicly released file.

For more info:

• https://wiki.mozilla.org/Security/Binary_Transparency

This is generated as part of the official publishing workflow using fdroid btlog. The canonical location for this data is:

• https://gitlab.com/fdroid/f-droid.org-transparency-log