Optional settings for fields

[jankatins]

Some things would in my opinion be useful:

• Currently only predefined metrics are available but not the underlying columns for these metrics. Sometimes it would be nice to also use metrics in groupers and so on: it would be nice if there could be a way to not hide a column which is the base for a measure.
• Metabase has a field type, so it would be nice if this could be set based on what is in mara-schema. E.g. if mara-schema would gain a Type.QUANTITY How to model measure as an attribute (and bug for attributes of Type.ID) mara-schema#8 it would be nice if this could be mapped to the metabase field type 'Quantity' so these columns are not automatically set to 'Category' in some cases.

As mara-schema should probably not be specific to the viz layer after it, maybe all attributes/measures in mara-schema could get an optional label: Optional[dict] = None so that this could be passed through? E.g. labels={'mara-metabase':{'hide': False, 'field_type': 'Quantity'}}?