PTFM-11133 DXEnvironment improvements.

* DXEnvironment.Builder supports initializing a new builder from an
  arbitrary JSON file or an existing environment.
* DXEnvironment.Builder#setBearerToken lets you set the bearer token (as
  a String) instead of having to build, and then set, the entire
  security context JSON.
* DXEnvironment.Builder supports setting, and DXEnvironment supports
  getting, all the other fields that are stored, too.