TDL-21365 Update custom field handling and custom fields schemas

[dsprayberry]

Description of change

• Modifies custom field handling to ensure formatting is consistent with how the Mambu API functions.
• Updates schemas for streams that use custom fields to ensure persistence of correct formatting.

Manual QA steps

• validated with sandbox account / integration

Risks

• All streams using custom fields will cease to persist the field_set_id, id, and value keys and will instead persist new keys and values for each of their source custom fields. Those key value combinations can be arrays, which could increase the level of nesting / quantity of sub-tables downstream.

Rollback steps

• revert this branch