* Add Validation to HTTP Inbound
* Pull a validation functionality from `WebFluxInboundEndpoint` to its
super class `BaseHttpInboundEndpoint` making a validation available for
the `HttpRequestHandlingEndpointSupport` as well
* Do the same for the `validator()` option in DSL for the
`HttpInboundEndpointSupportSpec`
* Add `validator` XML attribute for both HTTP and WebFlux inbound
endpoint XSDs
* Test parsers for a new `validator` option
* Document validation in the `http.adoc`
* Apply some polishing in the `http.adoc`, as well as in the
`HttpRequestHandlingEndpointSupport` JavaDocs
* Clarify in `webflux.adoc` that validation is applied for the
`Publisher` items before the payload is finally built for the message to
send.
* Add WebFlux into the table of endpoints in the `endpoint-summary.adoc`
* * Remove unused imports
* Fix `SimpleMessageListenerContainerSpec` for deprecated `txSize` option
c18c2e2141