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: I52d0d0554973714189a6e1013d026f96503238b6