Infer key type from public key and private key bytes

[robdefeo]

Is your feature request related to a problem? Please describe.
Mailchain requires private keys to, decrypt and sign transactions. To add a private key to mailchain the used needs to include the private key and key type. It is not expected that users understand what key type they have.
While ethereum and other protocols support a single key type, substrate supports, ed25519, sr25519, and secp256k1.

Describe the solution you'd like
Create a method that can determine the key type from the public and private key bytes combination.

• create method in crypto/multikey
• method signature GetKeyKindFromBytes(publicKey []byte, privateKey []byte) (crypto.PrivateKey, error)
• check each key type to see if the private and public keys match
  • create PrivateKey from []byte
  • compare output of .Bytes() to publicKey []byte
• check all candidate key types
• if a single match return crypto.PrivateKey for the matching key type
• use "github.com/mailchain/mailchain/crypto" package
• no match return error
• multiple match return inconsistent/inconclusive error

Additional context
More key types will be added in the future

[asnimansari]

Shall I do this ?

[olimpias]

Hi @robdefeo, could you assign the task to me? Thanks!

[robdefeo]

closed by #535