Use trait for HMAC

[seanaye]

I am using this as a wasm module in js server runtime. I think changing the HMAC signing to a trait based approach would allow wasm binaries to leverage window.crypto for HMAC signing instead of the hmac crate. This would reduce bundle size and increase compat wit wasm.

Will make PR for this

[paolobarbolini]

Sounds good. I wander whether it'd be better to go for a trait or instead have a Cargo.toml feature for deciding which crypto backend to use.