feat(uptime): Add method, headers and body columns to UptimeSubscription

[wedamija]

This adds columns that allow users to customize which method their checks are sent with, as well as custom headers and body.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/uptime/migrations/0012_uptime_subscription_request_fields.py ()

--
-- Custom state/database change combination
--
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "body" text NULL;
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "headers" text DEFAULT '{}' NOT NULL;
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "method" varchar(20) DEFAULT 'GET' NOT NULL;

[wedamija]

This PR has a migration; here is the generated SQL for src/sentry/uptime/migrations/0012_uptime_subscription_request_fields.py ()

--
-- Custom state/database change combination
--
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "body" text NULL;
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "headers" jsonb DEFAULT '{}'::jsonb NOT NULL;
ALTER TABLE "uptime_uptimesubscription" ADD COLUMN "method" varchar(20) DEFAULT 'GET' NOT NULL;

On second thought, I should use a text field so that we control how the json is serialized. I'm not sure we'll be guaranteed the ordering we provided