Make local decoding of TTN messages optional

Decoding TTN messages locally requires the TTN decoders to be present at a specific location on the filesystem relative to the broker repo. The way these decoders are named and found is not useful for other organisations.

The ttn_processor should not do local decoding by default and this should be configurable via the .env file.

The ttn_decoder container should exit if local decoding is not in use.