[request] Use Hasura API as a cron trigger webhook

[jflambert]

The new 1.3.0 cron triggers feature has a "Webhooks" text box where users must input an endpoint and a payload. However the Hasura API is incompatible with the metadata included in cron triggers payloads (see #5438 for screenshots).

I think it would make sense to have a checkbox to enable external endpoints, but by default use the "local" hasura API endpoint. This way I could make administrative calls through run_sql (replacement for pgagent!) or automated remote schema registration.

[treystout]

I see that a spec has been merged to help support this feature (https://github.com/hasura/graphql-engine/blob/master/rfcs/transforms.md) what does this mean for this feature?