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

.github/workflows/ci.yml

@@ -77,6 +77,8 @@ jobs:
     timeout-minutes: 10
 
     env:
+      PLAID_CLIENT_ID: foo
+      PLAID_SECRET: bar
       DATABASE_URL: postgres://postgres:postgres@localhost:5432
       RAILS_ENV: test