Allow custom name for tracked functions

### Is your proposal related to a problem?

Tracked functions cannot be given a custom name. (Hasura 2.0.6 and PostgreSQL 13.4)

### Describe the solution you'd like

Tracked functions should have a "Custom GraphQL Root Field" section in the "Modify" panel.

### Describe alternatives you've considered

I could rename the source function the way I want it to appear in graphql, but at the cost of rewriting other code.

### If the feature is approved, would you be willing to submit a PR?

Sorry no.

Start with the tracked-functions Modify panel and the GraphQL naming path it uses. Confirm how tracked PostgreSQL functions are currently exposed, then define the Custom GraphQL Root Field behavior so the exposed name can differ from the source function name. Done means the custom name can be configured without renaming the database function; no files or tests are named in the issue.

Written by the indexing model from the issue text.