Add signature and key abstraction for anoncreds

[mikelodder7]

This adds traits that allow for abstracting the keys, signatures, and proofs to allow for more flexibility than hardcoded structs.

[swcurran]

Ran cargo test and got a build error. It’s the same one that failed in the CI tests above. Back to you @mikelodder7

swcur@docket:~/repos/anoncreds-v2-rs (mikelodder7/main)$ cargo test
    Updating crates.io index
  Downloaded vsss-rs v4.0.0-rc2
  Downloaded blsful v2.5.4
  Downloaded 2 crates (702.3 KB) in 1.03s
   Compiling vsss-rs v4.0.0-rc2
   Compiling blsful v2.5.4
   Compiling credx v0.1.0 (/d2/repos/anoncreds-v2-rs)
error[E0599]: no function or associated item named `init` found for struct `pok_signature::PokSignature` in the current scope
  --> src/knox/ps/verifier.rs:64:29
   |
64 |     let res = PokSignature::init(signature, &pk, &proof_messages, &mut rng);
   |                             ^^^^ function or associated item not found in `PokSignature`
   |
  ::: src/knox/ps/pok_signature.rs:12:1
   |
12 | pub struct PokSignature {
   | ----------------------- function or associated item `init` not found for this struct

For more information about this error, try `rustc --explain E0599`.
error: could not compile `credx` (lib test) due to previous error
warning: build failed, waiting for other jobs to finish...