rest: Use ParamValidator library, add BodyValidator

rest: Use ParamValidator library, add BodyValidator

Parameter validation is based on parameter definitions like those in the
Action API, using the new ParamValidator library. Handlers should use
the provided Handler methods to access parameters rather than fetching
them directly from the RequestInterface.

Body validation allows the handler to have the (non-form-data) body of a
request parsed and validated. The only validator included in this patch
ignores the body entirely; future patches may implement validation for
JSON bodies based on JSON schemas, or the like.

Bug: T223239
Change-Id: I3c37ea2b432840514b6bff90007c8403989225d5