This commit adds SSE-C (Server-Side Encryption - Customer) support to
the B2 native backend. The server uses a customer provided AES-256 key
to encrypt the files when you upload them to the bucket, and then it
discards your key from the servers RAM after you're done uploading.
The option names and descriptions are based off the S3 backend
implementation as the way S3 and B2 does SSE-C is pretty similar.
Fixes#6585
94deb6bd6f