feat: add support for public exponent 3

[madztheo]

Description

The PKCS#1 v1.5 signature verification in the library was done assuming the exponent used for the public key is 65537. While this is the most widely used exponent, the exponent 3 is also in use, notably in some electronic passports. So this PR aims to add the support for this exponent in addition to 65537.

Problem*

Unability to verify signature using 3 as public exponent.

Summary*

The function verify_sha256_pkcs1v15 now takes in a new argument to specify the exponent. The exponent can be either 65537 or 3 if any other number the verification will fail straight away.

pub fn verify_sha256_pkcs1v15(_: Self, instance: BNInstance, msg_hash: [u8; 32], sig: BN, exponent: u32) -> bool

A test case was also added for a 2048-bit RSA signature using 3 as a public exponent.

The signature gen Rust script also takes in an exponent argument (defaulting to 65537) to generate a sample signature with a custom exponent.

Additional Context

The public exponent 3 is required to verify the signatures of some passports.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[michaeljklein]

LGTM