ext_crypto_sr25519* externals

[kogeler]

1. ext_crypto_sr25519_public_keys Returns all sr25519 public keys for the given key id from the keystore.
2. ext_crypto_sr25519_generate Generates an sr25519 key for the given key type using an optional BIP-39 seed and stores it in the keystore.
3. ext_crypto_sr25519_sign Signs the given message with the sr25519 key that corresponds to the given public key and key type in the keystore.
4. ext_crypto_sr25519_verify Verifies an sr25519 signature. Only version 1 of this function supports deprecated Schnorr signatures introduced by the schnorrkel Rust library version 0.1.1 and should only be used for backward compatibility.

Aha! Link: https://soramitsucoltd.aha.io/requirements/PRE-43-5