Add RS256 support

[paweldomas]

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Adds basic support for RS256 signature verification. The 'key' should be passed as a string which contains public key in PEM format for example:

-----BEGIN PUBLIC KEY----- MFwwDQYJKoZIhvcN....................7ogs8j2XThEFa1iPooa RPE/2+a8z9N1+8d9a....................Ncijm15Z7y/psCAwEAAQ== -----END PUBLIC KEY-----

It would be great to have a new release available as lua rock if this PR gets merged eventually, thanks !

[EyMaddis]

I am not sure of the inner workings of local pubkey = crypto.pkey.from_pem(key), but this might be leading to the the latest security vulnerability: https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/

Right now this is not the case as only HMAC based certificates are allowed, but once public keys are used, this has to be considered.