Support OS-provided crypto libraries/APIs

[clehner]

Re: https://github.com/spruceid/didkit/issues/16, https://github.com/spruceid/didkit/issues/16#issuecomment-748346141

Performance: an OS may provide access to cryptographic hardware that is faster than using our own crypto dependencies on the CPU.

Security and interop: an OS may provide access to manage and use keys where the private key material is not exposed to the application (including ssi). This could be considered a special case of https://github.com/spruceid/ssi/issues/53.

Using OS-provided crypto libraries could be done with feature flags as in https://github.com/spruceid/ssi/issues/52, and/or or by detecting the library at runtime (dynamic linking), with fallback to our compiled code.