Function rsa_signature 

pub fn rsa_signature(private_key_pem: &str, data: &str) -> Result<String>

Signs data using RSA PKCS#1 v1.5 SHA-256 with the provided private key in PEM format.

Errors

Returns an error if:

• data is empty.
• private_key_pem is not a valid PEM-encoded PKCS#8 RSA private key or cannot be parsed.
• Signature generation fails due to key or cryptographic errors.