Enable selecting cryptography backend in the CLI

[alilleybrinker]

This requires #190 be implemented first.

Right now, the CLI can only use the RustCrypto cryptography backend, but ideally the selection of backend would be up to the user. Once support for conditionally compiling additional backends is added to the omnibor crate, we can add the same flexibility in the CLI.

The CLI does have one additional wrinkle that we must decide how to expose control for linking to these backends; in particular on some platforms users may want to be able to opt in to use their system-provided OpenSSL.