Implement DataHandler plugin in GDS

[thomas-bc]

F´ Version
Affected Component	GDS

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Feature Description

A common use-case for the GDS is "I want to do extra stuff with the downlinked data" (e.g. send it to some other place, monitor for things etc.)
This is currently demonstrated with fprime_gds_client_sample_script.py, but could be made even simpler with the new plugin system

We should develop a plugin that lets you define a DataHandler, and hook it into the system

Rationale

Extend GDS