feat: Contactable Inboxes API (#2101)

- Add endpoint which lists inboxes through which a contact can be contacted
- Conversation creation API auto-creates contact_inbox for specific channels [ Twilio, email, api]
- Ability to send the initial message payload along with the conversation creation
- Fixes #1678 ( issue saving additional attributes for conversation )

app/builders/contact_inbox_builder.rb

@@ -0,0 +1,41 @@
+class ContactInboxBuilder
+  pattr_initialize [:contact_id!, :inbox_id!, :source_id]
+
+  def perform
+    @contact = Contact.find(contact_id)
+    @inbox = @contact.account.inboxes.find(inbox_id)
+    return unless ['Channel::TwilioSms', 'Channel::Email', 'Channel::Api'].include? @inbox.channel_type
+
+    source_id = @source_id || generate_source_id
+    create_contact_inbox(source_id) if source_id.present?
+  end
+
+  private
+
+  def generate_source_id
+    return twilio_source_id if @inbox.channel_type == 'Channel::TwilioSms'
+    return @contact.email if @inbox.channel_type == 'Channel::Email'
+    return SecureRandom.uuid if @inbox.channel_type == 'Channel::Api'
+
+    nil
+  end
+
+  def twilio_source_id
+    return unless @contact.phone_number
+
+    case @inbox.channel.medium
+    when 'sms'
+      @contact.phone_number
+    when 'whatsapp'
+      "whatsapp:#{@contact.phone_number}"
+    end
+  end
+
+  def create_contact_inbox(source_id)
+    ::ContactInbox.find_or_create_by!(
+      contact_id: @contact.id,
+      inbox_id: @inbox.id,
+      source_id: source_id
+    )
+  end
+end