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/models/provider/plaid_sandbox.rb

@@ -3,6 +3,19 @@ class Provider::PlaidSandbox < Provider::Plaid
 
   def initialize
     @client = create_client
+    @region = :us
+  end
+
+  def create_public_token(username: nil)
+    client.sandbox_public_token_create(
+      Plaid::SandboxPublicTokenCreateRequest.new(
+        institution_id: "ins_109508", # "First Platypus Bank" (Plaid's sandbox institution that works with all products)
+        initial_products: [ "transactions", "investments", "liabilities" ],
+        options: {
+          override_username: username || "custom_test"
+        }
+      )
+    ).public_token
   end
 
   def fire_webhook(item, type: "TRANSACTIONS", code: "SYNC_UPDATES_AVAILABLE")