Add a Rake task to sync existing schools to SF

While the code in this branch will update sales force on create or
update, there is some work required to back-fill existing data into
Salesforce. This commit provides a Rake task, patterened after that in
`clubs-api` that will do this.

Inspired by:

https://github.com/RaspberryPiFoundation/clubs-api/blob/main/lib/tasks/salesforce_sync.rake

Author: fspeirs

lib/tasks/salesforce_sync.rake

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+namespace :salesforce_sync do
+  desc 'Sync all Schools to Salesforce'
+  task school: :environment do
+    School.pluck(:id).each do |school_id|
+      Salesforce::SchoolSyncJob.perform_later(school_id:)
+    end
+  end
+end