[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.
Why is this part of the `Credentials`? Normally the server location would be kept elsewhere, and you would use `DomainRestrictions` to find credentials matching a given location.