Make ProcessingReport implement MessageProvider

We want the validation processor to provide reports on output. And the Processor
interface requires that inputs and outputs implement this interface.

Maybe it shouldn't and this enforcement should be done at the ProcessingChain
level instead...

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Create skeleton classes for validation processor and keyword validators

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>