db: update webhookpayload

[joseph-sentry]

I add the destination_type field, make the region name nullable and add a constraint so if the destination_type == SENTRY_REGION then the region_name must be set to a value.

These are the equivalent database changes for a following change that will implement webhook forwarding to Codecov using the hybrid cloud webhook forwarding system. Webhooks meant to be forwarded to Codecov will have differing logic that runs to forward them which is why I'm adding the destination_type field so we can distinguish them.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/hybridcloud/migrations/0022_webhook_payload_update.py

for 0022_webhook_payload_update in hybridcloud

--
-- Add field destination_type to webhookpayload
--
ALTER TABLE "hybridcloud_webhookpayload" ADD COLUMN "destination_type" varchar DEFAULT 'sentry_region' NOT NULL;
--
-- Alter field region_name on webhookpayload
--
ALTER TABLE "hybridcloud_webhookpayload" ALTER COLUMN "region_name" DROP NOT NULL;
--
-- Create constraint webhookpayload_region_name_not_null on model webhookpayload
--
ALTER TABLE "hybridcloud_webhookpayload" ADD CONSTRAINT "webhookpayload_region_name_not_null" CHECK (("destination_type" = 'sentry_region' AND "region_name" IS NOT NULL)) NOT VALID;
ALTER TABLE "hybridcloud_webhookpayload" VALIDATE CONSTRAINT "webhookpayload_region_name_not_null";