[FEATURE] Airtable sources should optionally emit all fields for new / updated records, including fields that hold no values

From Slack:

> Re: Airtable (and possibly other Pipedream-built) triggers: Is there a way to force ALL columns/fields -- even those with null values -- to be returned in that Pipedream object?
We are syncing new/updated Airtable data to Snowflake as JSON blob versions of that returned object and it seems that the Pipedream's Airtable new/updated trigger is ignoring null fields when creating that returned object - sometimes the object ay be 5 fields, sometimes 7 etc. And the Snowflake team say they'd love to get a consistent set of the full fields every time.

Airtable only includes fields that hold valid values in their list records API response, but it's possible the Airtable source could use the [metadata API](https://airtable.com/api/meta) to list the table's schema, decorating the emitted event with the field names (and null values) for other fields.