Enhancement: Update the Backblaze B2 library

We've updated the library we're using for accessing the Backblaze B2 service to
0.5.0 to include support for upcoming so-called "application keys". With this
feature, you can create access credentials for B2 which are restricted to e.g.
a single bucket or even a sub-directory of a bucket.

https://github.com/restic/restic/pull/1901
https://github.com/kurin/blazer
