Include form field labels and hints in form_schema

1. What is the general goal of the feature?
Originally suggested here by @dmenne

What: Provide form labels and hints in all supported languages to consumers of ODK Central data, e.g. though ruODK.

How: The output of ODK Central's form schema (api docs on form schema) could add labels and hints.

    "name": "age",
    "path": "/age",
    "type": "int",
    "label": {"en": "Age", "de": "Alter"},
    "hint": {"en": "...", "de": "..."}

Note on example schema above: Nesting languages inside label and hint would provide a stable schema at the top level and push the changing number of languages to the second nesting level.

2. What are some example use cases for this feature?
As a data analyst building data products directly from ODK Central data, I want to use human readable form labels and/or hints in the languages supported by the form.

3. What can you contribute to making this feature a reality?
Support in ruODK::form_schema()