[WiP] - JENKINS-24702

convert legacy credentials stored as plain text (including Oauth token :-\) to use credentials-plugin.
Maybe AccessTokenCredential is generic enough for this to be defined by some oauth-credentials plugin, not sure how portable this is and if this would make sense.
Could be an option, but I prefer to offer a nicer UX : just get the generate token option where you set the credential first time.