ECDSA Support in signer

[jsdw]

Pushing #1052 over the finish line; thankyou @LGLO for your awesome addition :)

[jsdw]

One thing I still need to gain clarity on is that the SECP256K1 context was used in the sign, and the Secp256k1::signing_only() was used in from_seed(). The sp_core impl uses the Secp256k1::signing_only() in both cases (when no-std), and so for now I made this code do the same in the hopes that this also provides wasm support out of the box (the impl looked like it would). @LGLO perhaps you could provide some insight into your choices here?

I added a wasm test for escda sign/verify anyway so hopefully that will catch any wasm issues there.

I also think it's prolly a good idea to have a feature flag to enable each of sr25519 and ecdsa now that there are two impls, so I'll probably also add that before getting this reviewed etc!

[jsdw]

The global signing context seems to work fine in WASM, so I switched to using that (as Substrate does when "std" is enabled. I also added feature flags so that people can opt-out of impls they don't need, a check for them in CI and a small doc test

[LGLO]

@jsdw Thank you very, very much for taking care of it!

@LGLO perhaps you could provide some insight into your choices here?
Unfortunately, I can not. It seems that discrepancy came from my lack of awareness.
I think using SECP256K1 global context is OK. This code doesn't depend on randomness. I think the problem in no-std environment would be with context randomness initialization.

[jsdw]

Thanks for your reply and contribution! That makes sense; I think I'll have a pass over making this crate no_std soonish and then might need to revisit eg the global context thing which seems to require "std", but for now supporting std and wasm is good enough for us so it's all gravy :)