Expose cryptography backend features in the omnibor crate

[alilleybrinker]

We've recently added support for additional cryptography backends (namely, BoringSSL and OpenSSL) in the gitoid crate, in addition to the already-included RustCrypto backend. However, those new backends aren't exposed in the omnibor crate, which means users of the OmniBOR crate are still limited to just RustCrypto.

These features should be exposed in the omnibor crate in the same way they are in the gitoid crate, in a new submodule for cryptography backends, and with crate features whose names match those in the gitoid crate.

Note that this does complicate conditional compilation flags, and requires testing more combinations. In particular, we'll need to add code, as there is in the gitoid crate, to cover the case that no cryptography backend is enabled at compile time.