source: facebook-marketing: improve check method for custom insights

## Tell us about the problem you're trying to solve
Incremental syncs fails on custom insights streams when not all parts of the composite PK are listed as fields in the custom insights fields.

## Describe the solution you’d like
improve the check method and add logic so that if a customer has a custom insight defined, they need to at least specify the account_id, date_start, and ad_id, otherwise we won't be able to build the composite key.

## Describe the alternative you’ve considered or used
Could we handle this for the user? I.e. if custom insights are defined, we add these fields to their custom insights fields if not already specified? In that case maybe these aren't even selectable by the user, they get populated by default and are non-removeable so long as there are >0 fields specified

## Context
https://github.com/airbytehq/oncall/issues/1304