Revert "Remove buildpack_name and buildpack_guid from app usage events."

This reverts commit d5254becab0607830fc6e1945b5e40bf8dfefaa6.
Create a 'BUILDPACK_SET' app usage event

App usage events are currently created every time the desired state of
an application goes from STARTED to STOPPED or when the desired instance
count or memory allocation of a STARTED app changes. A requirement has
been raised to add buildpack information to these events but the
buildpack data is not always known at the time the desired state of an
app transitions to STARTED.

Since there are hard requirements to have events associated with the
existing STARTED and STOPPED states created without introducing windows
where events are duplicated or recorded out of order and to ensure that
events are immutable, the tactical resolution was to introduce a new
state called BUILDPACK_SET that represents a simple buldpack update.
While not ideal, it surfaces the information without breaking existing
code that depends on the STARTED and STOPPED events.

This change records app usage events containing buildpack information
when staging completes.

[#69196650]
Merge pull request #230 from sykesm/staged-event

Summation:  This adds a 'BUILDPACK_SET' state to app update events, and creates new events with this state whenever staging for an app completes.