Add boilerplate for sending SMS via Reach

This works in conjunction with the new SMS provider stub [^1].

Local testing:

- Run the migrations to add Reach as an inactive provider.
- Activate the Reach provider locally and deactivate the others.

      update provider_details set priority = 100, active = false where notification_type = 'sms';
      update provider_details set active = true where identifier = 'reach';

- Tweak your local environment to point at the SMS stub.

      export REACH_URL="http://host.docker.internal:6300/reach"

- Start / restart Celery to pick up the config change.
- Send a SMS via the Admin app and see the stub log it.
- Reset your environment so you can send normal SMS.

      update provider_details set active = true where notification_type = 'sms';
      update provider_details set active = false where identifier = 'reach';

[^1]: https://github.com/alphagov/notifications-sms-provider-stub/pull/10

README.md

@@ -29,6 +29,7 @@ export NOTIFY_ENVIRONMENT='development'
 
 export MMG_API_KEY='MMG_API_KEY'
 export FIRETEXT_API_KEY='FIRETEXT_ACTUAL_KEY'
+export REACH_API_KEY='REACH_API_KEY'
 export NOTIFICATION_QUEUE_PREFIX='YOUR_OWN_PREFIX'
 
 export FLASK_APP=application.py
@@ -45,6 +46,7 @@ Things to change:
 ```
 notify-pass credentials/firetext
 notify-pass credentials/mmg
+notify-pass credentials/reach
 ```
 
 ### Postgres