Improve use and documentation of using different cryptographic libraries

[clehner]

ssi supports using different libraries for cryptographic functions: spruceid/ssi#217 and may add more: spruceid/ssi#216. This optionality should be exposed to DIDKit users, and documented. For example, how does one build didkit-cli and didkit-wasm using ed25519-dalek.

• Relevant commands could be listed in the readmes
• The Makefile could be extended to build with features selected using Make variables.

[bumblefudge]

Maybe start with the Rust docs, then add commands to other interfaces?

[bumblefudge]

As for the makefile, I'm a huge fan of commenting-out alternatives-- makes the documentation much easier for someone else to write :D