[Maps] clean-up IDynamicStyleProperty

[nreese]

Creating the IDynamicStyleProperty interface has highlighted some uglyness in the existing API. See #59707 (comment) for details.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

getFieldName: usages in dynamic_[color|label|orientation|size]_form
It also highlights an unnecessary double lookup here:

const getField = () => {
    const fieldName = styleProperty.getFieldName();
    if (!fieldName) {
      return null;
    }

    return fields.find(field => {
      return field.name === fieldName;
    });
  };

That is not actually a double look up. The field returned from DynamicStyleProperty.getField is an IField. The fields passed into DynamicColorForm do not have a type defined but look like { label, name, origin, type }. This lookup is needed to pull the correct object from the fields array and the contents of the field array are not whats returned from DynamicStyleProperty.getField