Field selection for /extras extension point

[nuest]

The /extras endpoint should get a possibility for subsetting the top level fields to allow handling a large number of extension points or to handle extension points which provide large responses.

Full extra information: ../timeseries/id_001/extras:

{
    "fieldA":"somedata",
    "fieldB":1.1,
    "fieldC":{
        "subfields":true,
        "subfield2":42,
        "evenmorestuff": { [...] }
    }
}

Selected extra information: ../timeseries/id_001/extras?fields=fieldA,fieldB

{
    "fieldA":"somedata",
    "fieldB":1.1
}

The name should be discussed, could also be "subset", or "select".

[ridoo]

Nice one! Yes, this would make sense but also for the whole API.

[nuest]

Yes, this would also work as a common query parameter.

[ridoo]

I created issue #169 because of the comment above. this issue is closed.