Allow for disabling client-side validation in V4 Post Policy #1178

When calling validateFields; it checks to make sure provided input field keys are part of a known list. This is restrictive on input and would require an update if we introduce more fields in the future (which is possible).

Follow-up:

• Does the server return a useful error?
• Is the error state opaque such that the server can't return a better error?
• If the server can be improved should we advocate that?
• If the server error can only be opaque, do we want to introduce a strict/relaxed validation option? strict: throw exception or relaxed: log a warning for customer