libsnapshot: Add Parser base class

Add in Parser base class and refactor code to work with it. Base class
will have ops() method which returns a vector of v3 operations.
v2_parser translates it's v2 operations to v3 operations with this
method.

Bug: 307452468
Test: Test with critical OTA paths?
Change-Id: Iccc9755a892911a1638f5b62d7d6a2402c68ab20