Improve use and documentation of using different cryptographic libraries

[clehner]

ssi supports using different libraries for cryptographic functions: https://github.com/spruceid/ssi/issues/217 and may add more: https://github.com/spruceid/ssi/issues/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.