Add ECIES encryption support to subxt-signer

[hitchhooker]

Summary

• Adds encrypt / decrypt methods to sr25519::Keypair using schnorrkel's ECIES module
• New ecies feature flag in subxt-signer
• Includes example: cargo run -p subxt-signer --features ecies --example ecies

Depends on: paritytech/schnorrkel#116 (schnorrkel needs to publish with the ecies feature before this can use a crates.io version)

Test plan

• Example runs successfully with dev accounts (Alice/Bob)
• Wrong key and wrong context correctly fail decryption
• CI validation
• Review by maintainers

[jsdw]

I wonder whether this feature would be better called "sr25519-ecies" since it seems specific to that (and then it would be a stronger hint that this enables some feature in the sr25519 module)

[jsdw]

Until the original non-fork version of this has the relevant feature and is available on crates.io, we won't be able to merge this: git URLs prevent publishing, and I would not want to deviate from the Parity crate, in large part for security reasons :)

[jsdw]

Thankyou; offhand this looks good to me! Will need to wait for the schnorrkel crate to be published though.

My only general reservation is that the subxt-signer crate is really just some helpers to sign/check transactions which plays nicely with subxt. I would thus be hesitant to expand the features too much beyond this, and would generally encourage people to create alternate crates for more complex use cases. That said, this addition looks fairly small and simple enough so I think it would be ok to add (pending the feature making it to the Parity crate)!

[cla-bot-2021]

User @claude, please sign the CLA here.