Split payload metadata logic from DeltaPerformer into its own class.

DeltaPerformer have code for both parsing payload and performing
operations. This change moves parsing payload header and validating
metadata signature to a new class PayloadMetadata so that
DeltaPerformer can focus on performing.

We will also have new code in another class that will use the new
PayloadMetadata class to parse payload metadata.

Bug: 65283633
Test: update_engine_unittests
Change-Id: Ie20b84713a0c66867a1de9d3d0cc29d0189b3c97