Introduce a general purpose RestDocumentationHandler

[wilkinsona]

RestDocumentationFilter and RestDocumentationResultHandler currently duplicate logic. Broadly speaking they:

• Create an OperationRequest from the native request
• Create an OperationResponse from the native response
• Create an Operation from the OperationRequest and OperationResponse
• Call all the snippets to document the operation

This logic could be handled by a general purpose RestDocumentationHandler to which RestDocumentationFilter and RestDocumentationResultHandler can delegate.