[Feature] Support `starknet` platform

[nykma]

Description

starknet's address format: 0xHEXSTRING_32_BYTES (compressed public key?).

Which is different from ethereum's 0xHASH_OF_PUBKEY_20_BYTES.

So starknet platform is not duplicated with current ethereum
platform, and worth supporting.

Expected outcomes

validators/starknet should be provided, both implementation and unit tests.

Signature verifying procedure should only rely on signatures generated
by starknet identity in starknet's wallet app.

You should figure out which signing algorithm is popular among
starknet community and wallets, and use it in our verifying
workflow.

You may take validators/ethereum as a sample.