Change GraphQL comment for user defined function and arguments

It would be very valuable for us to be able to define custom comments for tracked functions in the metadata.

Ideally, it should be possible to add custom comments on the following:

* The function itself: Currently the auto-generated comment reads something like:

```
execute function "my_function" which returns "my_schema.my_table"
```

A user-provided comment should entirely override the auto-generated comment, without disclosing what the underlying pg function is.

The auto-generated comment should ideally replace the `my_schema.my_table` with any custom GraphQL root name provided by the user.

* The function arguments: Currently, there is no way to add comments to those and there is no auto-generated comment either.