Add missing `bs` crate operations

[dhuseby]

https://github.com/cryptidtech/bs/tree/main/crates/bs/src/ops

We still need to implement the following operations:

• Verify — iterate over a plog's entries and verify each one
• Fork — create one or more child plogs from a single parent plog
• Merge — create a new plog with more than one parent plog
• Close — rotate keys to null and/or make the lock script impossible to solve to prevent future updates
• Get — this gets the most recent value of a given key-path

I would like the bulk of the provenance log operations to be implemented in the bs crate. This is also a great place to define traits for encrypting, decrypting, signing, verifying, rotating keys, etc. This is the crate that will be used by the BetterSign cli tool as well as the BetterSign peer. It should contain all of the BetterSign-specific plog operations and traits.