Get token using jwt / private key

[drzraf]

If a token expired, two ways to get a fresh one are provided. Either using getPasswordAccessToken either by getClientCredentialsAccessToken which relies upon client client's id and secret: https://github.com/eventfarm/restforcephp/blob/0f3589b2b00fb1d3f1b3f4e71668a02feb4bf6e9/src/Rest/OAuthRestClient.php#L142

I think it's missing the 3rd one, using a private key. Would you accept a MR which would basically add a (?string) $privateKey parameter on the model of $clientId & co and a getJwtAccessToken($client_id, $private_key, $validity = 3600) method?

Please note that OAuthRestClient is final.