Add ecdsa support to signer

[LGLO]

This adds support for ECDSA in signer, so it allows to submit transactions to nodes were ECDSA is used as blocks signing scheme.

If there is any interest, I'm happy to work on this PR, please let me know if:

• message hashing algorithm should be parametrized (instead of hardcoded blake2b)
• some common parts, that are copy-paste from signer::sr25519 should be extracted (to utils.rs ?)
• example should stay (I'm not convinced there is much added value in it)

[cla-bot-2021]

User @LGLO, please sign the CLA here.

[jsdw]

Ooh thank you for your contribution; it'd be great to get ECDSA support added!

I'll have a more thorough look over this in the next couple of days!

Offhand, the example can probably go for now (or alternately be added to the book alongside the sr25519 example; all examples are included somewhere in the book). Also, we have no real Nix knowledge on the team and so the flake files should go as well (we wouldn't be able to keep them uptodate or whatever so would rather keep such things out of the repo). The implementation itself looks good though; I'll have a think about code reuse but for now I think it's all good on that front! Running cargo fmt and cargo clippy --fix might remove the other CI issues :)

[jsdw]

I'll fix up the issues etc in #1064 and get this merged (I haven't got a good setup for pushing commits to fork branches which is the only reason I made a new one); thankyou for your awesome contribution :)