Update detected buildpack on diego staging path

Use buildpack_key form response message to update buildpack information
in the app.

[#69196650]
Add /v2/apps/:guid/env and bump CC version

-Endpoint exposes normal env vars and VCAP services
-Bump CC version to 2.4.0
-Update API docs to reflect changes
Merge pull request #233 from sykesm/diego-staged-event

Update detected buildpack on diego staging path