Implement S3-based BackupLocation

[ecton]

We should support backing up to an S3 bucket. by implementing BackupLocation for a type that supports S3.

We should consider move shared S3 code from bonsaidb-keystorage-s3 into a new package, bonsaidb-s3, if we find there's enough to warrant it. Having a single configuration to represent an S3 bucket location seems useful.

• Add new crate with a new type that implements BackupLocation: bonsaidb-backup-s3
• Examine if the configuration code (or any other code) in bonsaidb-keystore-s3 and bonsaidb-backup-s3 can be extracted into a shared crate, bonsaidb-s3