support VOLATILE function for computed field.

> I have a case where using a VOLATILE SQL function in a computed field would simplify things alot.

> I also noticed I can create the function as STABLE, add it as computed field, and then change the function to VOLATILE and it works just fine in hasura.

> But feels hacky and hasura complains on metadata reload. Also seem's there's not really much needed to support this except allowing VOLATILE function to be tracked, at least in my case.

> Will the work for this issue support this use case as well ?

_Originally posted by @karibertils in https://github.com/hasura/graphql-engine/issues/1514#issuecomment-711073631_

Any chance volatile functions for computed fields can be enabled? If I do the above trick the computed fields of a table, returned by a custom query function (`returns setof`) are correct. If my computed fields functions are `STABLE` they have the value from before the mutation occured.

No files, tests, or entry points are named. Start by locating computed-field tracking and the handling of PostgreSQL function volatility described in the issue; done means VOLATILE functions can be tracked without metadata reload complaints and computed fields reflect values after mutations.

Written by the indexing model from the issue text.