Implement Digest traits for SHA

[Natureshadow]

In order to use the hardware-accelerated SHA on ESP32 with external code that accepts implementations of the digest traits, it would be helpful to implement those on the Sha struct.

The implementation should probably be behind a digest feature flag in order to avoid the dependency if not needed.

[bjoernQ]

+1 - we should implement the crypto traits for all crypto-accelerators: https://github.com/RustCrypto/traits

but we will need to do what is worked on in #866 for them first