Added an optional extractMapping transform option to all metadata extractors to override the default one in the T-Engine.
In the case of the AGS AMP it extends the RFC822MetadataExtracter with its own class to specify a different set of document to system mappings. The class in the repo no longer does extractions, but is now used by the AsynchronousExtractor, which offloads extractions to T-Engines to obtain the mappings if it has been extended that are then passed to the T-Engine.
