When preparing for a license check on the Licenses page, a License

Provider can now provide a Session Token, which is an opaque value that

will be passed back during processing of a response. That allows

information to be temporarily stored and then retrieved for response

processing.
Responses are also no longer required to be JSON. A license server may

now provide any kind of text-based response, which may be processed by

the License Provider however needed.

Unit tests pass.
Tested this with the in-progress work in Power Pack.