Exercise on verifying ECDSA signatures

[tekkac]

Description

Users can sign messages using their stark_key as seen on the key pair exercise.
Cairo implements the signing in python and the verification in Cairo.
Provide an exercise that verifies a generated signature using the ecdsa builtin.

Resource:

• https://github.com/starkware-libs/cairo-lang/blob/master/src/starkware/crypto/starkware/crypto/signature/signature.py
• https://perama-v.github.io/cairo/examples/verify_ecdsa/

Acceptance criteria

• The exercise show how to securely verify an ECDSA signature.
• The contract compiles and tests pass
• An exercise patch is generated