Adds RSA key encryption and signature scheme

[HLRichardson-Git]

• This PR is for Issue #26

Description of the Change

Adds the Rivest–Shamir–Adleman (RSA) key encryption and signing schemes from FIPS 186-5 and padding schemes from RFC 8017

These can all be uses by a user by including #include <gestalt/rsa.h>, here is an example of using RSA to sign a message with the PSS padding scheme:

#include <gestalt/rsa.h>
#include <iostream>

int main() {
    RSA rsa({RSASecurityStrength::RSA_2048, RandomPrimeMethod::probable});
    PSSParams parameters = {HashAlgorithm::SHA1, RSA_MGFFunctions::MGF1};

    std::string messageToSign = "Hello, Gestalt!";
    std::string computedSignature = rsa.signMessage(messageToSign, parameters);

    std::cout << "Signature: " << computedSignature << std::endl;

    RSAPublicKey peerPublicKey = publicKey; // For the sake of example
    bool isValidSignature= rsa.verifySignature(messageToSign,  computedSignature, peerPublicKey, parameters);

    if (isValidSignature) std::cout << "Signature is valid!" << std::endl;

    return 0;
}

Updates the following documentation:

• README
• CHANGES

Future Additions

• Certificates are not considered for this PR, it would be best if we add functionality for users to import/ export RSA keys to and from Gestalt.
• PKCS#1v1.5 padding scheme for encryption and signing is not currently provided despite being the simplest of the three main padding schemes. It has a tricky leading zero which is proving challenging to use with out multi-precision library (GMP) which strips leading zeros.
• Optimizations. Generally the implementation of RSA is slow compared to ECDSA (which I know should be the case) but I want to lessen the difference in performance.

Verification Process

There are a lot new unit tests to test all current features of the RSA in Gestalt which can be found in tests/rsa/.

Release Notes

• Adds RSA Key Generation with provable or probable primes with key sizes of 1024 to 15360.
• Adds RSA key encryption with Raw or OAEP padding schemes.
• Adds RSA message Signing Scheme with Raw or PSS padding schemes.
• Refactors the BigInt class outside of ECC to its own file in tools/bigint/bigint.h to be used anywhere.
• Refactors the hash utilities in ECC to its own file in tools/hash_utils/hash_utils.h to be used anywhere.