Add start and stop commands to FileWriterPlugin

[GDYendell]

• Depends on Implemented command and requestCommands methods in the FrameProcessor. #359

• Implement "start_writing" and "stop_writing" in execute to replace hdf/config/write

• Return ["start_writing", "stop_writing"] in requestCommands

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To be tested alongside

• Update odin data adapter to expose commands #363
• Add introspection of commands in odin data applications DiamondLightSource/fastcs-odin#41

[jsouter]

Commenting so I can be assigned!