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

migrations/versions/0367_add_reach.py

@@ -0,0 +1,54 @@
+"""
+
+Revision ID: 0367_add_reach
+Revises: 0366_letter_rates_2022
+Create Date: 2022-03-24 16:00:00
+
+"""
+import itertools
+import uuid
+from datetime import datetime
+
+from alembic import op
+from sqlalchemy.sql import text
+
+from app.models import LetterRate
+
+
+revision = '0367_add_reach'
+down_revision = '0366_letter_rates_2022'
+
+
+def upgrade():
+    conn = op.get_bind()
+    conn.execute(
+        """
+        INSERT INTO provider_details (
+            id,
+            display_name,
+            identifier,
+            priority,
+            notification_type,
+            active,
+            version,
+            created_by_id
+        )
+        VALUES (
+            '{}',
+            'Reach',
+            'reach',
+            0,
+            'sms',
+            false,
+            1,
+            null
+        )
+        """.format(
+            str(uuid.uuid4()),
+        )
+    )
+
+
+def downgrade():
+    conn = op.get_bind()
+    conn.execute("DELETE FROM provider_details WHERE identifier = 'reach'")