Add security prices provider (Synth integration) (#1039)

* User tickers as primary lookup symbol instead of isin

* Add security price provider

* Fetch security prices in bulk to improve sync performance

* Fetch prices in bulk, better mocking for tests

test/models/account/holding_test.rb

@@ -58,7 +58,7 @@ class Account::HoldingTest < ActiveSupport::TestCase
     end
 
     def create_holding(security, date, qty)
-      price = Security::Price.find_by(date: date, isin: security.isin).price
+      price = Security::Price.find_by(date: date, ticker: security.ticker).price
 
       @account.holdings.create! \
         date: date,