adding thredds param to core

[lbesnard]

the ACORN handler needs to download data from the thredds server.
see
aodn/python-aodndata#43 (comment)

Depending on the environment, RC, Prod or stack, this needs to be somehow transparent to the aodndata package

[mhidas]

I don't think this really need to be in core. As @lwgordonimos suggests here, it's very easy to add it as a custom_param to the few specific handlers that need it.

[lbesnard]

in that case how do you make the difference between thredds-rc.aodn, thredds.aodn, thredds-stack ? if you add a custom_param, it means you have to create a different json file for different machines/environments. We don't do it for any other pipelines

[mhidas]

Yeah, sorry. I was just realising that. Never mind.

[Deleted user]

Resolved by #104