Introduce RSA function to js engine

Please implement RS256 (RSA Signature with SHA-256) encryption function to js engine. This encryption is required for creating integrations with some services (google cloud JWT signature in particular). Similar function already implemented with HS256 (HMAC with SHA-256) in task ZBXNEXT-7615

RSA encryption assume using PKCS #8 standard syntax for storing private key, so decision should be made about how to store it and use in js - file or string.