Rest: allow Handlers to disable body parsing.
Handlers should be able to opt out of automatic parsing of JSON body
content. This is inportantr in two cases:
- The handler wants to process the body as a stream
- The handler wants to use a custon BodyValidator.
Bug: T357025
Bug: T359149
Change-Id: Iac1ad0b723cbad4c79c7119ab0dca8a08b19fe32