Add support for plugins using `BATCH` message type

Once Meltano supports the PipelineWise taps and targets out of the box (#2357 (closed)), it would be great if we could also support Fast Sync when meltano elt is run with a supported pair of PipelineWise taps and targets:

Fast Sync is bypassing the Singer Specification for optimised performance. Primary use case of Fast Sync is initial sync or to resync large tables with hundreds of millions of rows where singer component would usually be running for long hours or sometimes for days.

See:

• https://transferwise.github.io/pipelinewise/concept/fastsync.html
• https://transferwise.github.io/pipelinewise/concept/replication_methods.html#fast-sync
• https://github.com/transferwise/pipelinewise/tree/master/pipelinewise/fastsync