New ContentNegotiationService and ContentMappingService SPIs for Restful Objects viewer

The ContentNegotiationService SPI allows multiple implementations to be installed, and each are checked in turn (chain of responsibility pattern). If any can generate a representation of the result then this is used rather than the default RepresentationService.

The framework provides one default implementation: ContentNegotiationServiceXRoDomainType that handles "application/...;profile=...;x-ro-domain-type=..." for either XML or JSON, and for the main representations (object, property, collection, detail, action-result)

As a further sub-SPI, the ContentMappingService is used by the afore-mentioned ContentNegotiationServiceXRoDomainType to attempt to map the result domain object to another object before rendering. Typically this is to map a domain object to a corresponding DTO.