Allow legacy fieldmask marshaling

[adriansmares]

References https://github.com/TheThingsIndustries/lorawan-stack/pull/3445

We should have a global option available in the plugin that allows us to marshal google.protobuf.FieldMask as objects ({paths: ["a.b.c","d.e.f"]} instead of strings ("a.b.c,d.e.f"}.

The current state of affairs is that field mask objects can be unmarshalled both from strings, and from objects, but marshaling always goes towards strings. This breaks our API continuity for webhooks and end device template conversion.

I would like to see an option, similar to std (std=true generates standard encoding/json marshalers), which enables this legacy behavior. The most appropriate name that I can come with is legacy_fieldmask_marshaling, but feel free to suggest otherwise.