[Okta] Supporting credentials in PEM format

[jamiehynds]

Within the Oauth2 configuration in our Okta integration is currently limited to private/public key pairs in JWK format. Okta generally recommends Privacy Enhanced Mail (PEM) for private/public key pairs, and we've also seen user requests to support PEM.

Can we update our integration to support PEM?

When complete, we also need to ensure [our documentation](Privacy Enhanced Mail) is updated, we we currently state:

"Generate your own Private/Public key pair in the JWK format (PEM is not supported at the moment) and save it in a credentials JSON file or copy it to use directly in the config."

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)