Plaid sync domain improvements (#2267)

Breaks our Plaid sync process out into more manageable classes. Notably, this moves the sync process to a distinct, 2-step flow:

1. Import stage - we first make API calls and import Plaid data to "mirror" tables
2. Processing stage - read the raw data, apply business rules, build internal domain models and sync balances

This provides several benefits:

- Plaid syncs can now be "replayed" without fetching API data again
- Mirror tables provide better audit and debugging capabilities
- Eliminates the "all or nothing" sync behavior that is currently in place, which is brittle

app/controllers/webhooks_controller.rb

@@ -6,7 +6,7 @@ class WebhooksController < ApplicationController
     webhook_body = request.body.read
     plaid_verification_header = request.headers["Plaid-Verification"]
 
-    client = Provider::Plaid.new(Rails.application.config.plaid, region: :us)
+    client = Provider::Registry.plaid_provider_for_region(:us)
 
     client.validate_webhook!(plaid_verification_header, webhook_body)
     client.process_webhook(webhook_body)
@@ -21,7 +21,7 @@ class WebhooksController < ApplicationController
     webhook_body = request.body.read
     plaid_verification_header = request.headers["Plaid-Verification"]
 
-    client = Provider::Plaid.new(Rails.application.config.plaid_eu, region: :eu)
+    client = Provider::Registry.plaid_provider_for_region(:eu)
 
     client.validate_webhook!(plaid_verification_header, webhook_body)
     client.process_webhook(webhook_body)