Sign and verify messages using sr25519 and ecdsa keypairs

[justkawal]

Tracking issues for support Sr25519 and ECDSA schemes:

Currently there's no Sr25519 equivalent library in Dart, limiting polkadart to ED25519 and ECDSA keys.
The polkadot-api javascript implementation also uses the rust code compiled to webassembly @polkadot/wasm-crypto.

Unfortunately we cannot assume a webassembly container in dart, which left us with two options:
1 - Implement Flutter and Dart native bindings (it still doesn't work for web targets).
2 - Rewrite schnorrkel in dart (very slow and unsafe, as dart doesn't provide memory safety that rust does).

• Support Sr25519 key derivation
• Support Sr25519 signed extrinsics
• Support ECDSA key derivation
• Support ECDSA signed extrinsics