Graphql-engine: support Postgres functions as Action handlers

[cpursley]

The Actions announcement indicates that Postgres functions can be used as Action handlers. However, I haven't been able to find docs covering this nor usage examples.

https://hasura.io/blog/introducing-actions/

[rikinsk]

Actions in its initial release will not be supporting PG functions. This functionality should be added in the coming releases.

Closing this issue for now.

[tirumaraiselvan]

Changing the title to track the feature.

[cpursley]

Is there an update on an expected release for Postgres Action handlers?