Make the various hash function crates optional?

[vdods]

In the SAID spec I only see reference to BLAKE3, and using that hash function seems to be the only behavior when calling SAD::compute_digest (my attempts to specify a different hash function in SelfAddressingIdentifier::new don't seem to produce the desired effect).

Would it be possible to make the various hash functions besides blake3 optional, so that dependencies can be minimized?

Maybe do the same with serde_json, serde_cbor, rmp-serde if that's possible?