Basic Snapshot Implementation in Pegasus.
Snapshots are a basic representation of a Resource, like an IDL, but with models included so that we can check
model backwards compatibility as well as Resource Compatibility. Snapshot compatibility checking replaces IDL
compatibility checking.
PegasusPlugin was not updated to use or create Snapshots, so external projects will continue to use IDLs
rather than Snapshots.

RB=173757
R=gogenc,jbetz,si-dev,kjin
A=gogenc,jbetz