Add support for secp256r1

[0xknwn]

🧐 Motivation

OpenZeppelin/cairo-contracts includes a number of utils to support secp256k1:

• types for publickey and signature
• traits for serialization, debug, eq
• a store to save the secp256k1 public keys in a contract in a packed format
• is_valid_eth_signature to check a secp256k1 signature is valid
• a collection of Cairo tests and a data set to validate the implementation of the above

These tools are used internally in OpenZeppelin/cairo-contracts to implement the Eth Account. They are also available for third party projects that uses your project as a library. We use them in our starknet-modular-account project.

We have implemented the same set of tools for secp256r1 that is also supported by the starknet corelib. We would like to submit a PR that them so that we can rely on them the same way we rely on your secp256k1 implementation.

📝 Details

See PR #988 for details about the implementation. We've done the adaptation from secp256k1 to secp256r1 but, except for the mapping between the functions and features the implementation is the same.