Leave out generated columns in mutations

### Is your proposal related to a problem?

This is related to #4633 as batch inserts to tables with generated columns always fail. However hasura:

1. Should not even generate input field for generated column, because passing any value would be rejected by postgres
2. Should not include those columns in the generated SQL, because as per #4633 it fails with >1 insert.

### Describe the solution you'd like

1. Do not generate input fields for generated columns in mutations
2. Generate SQL inserts that do not include generated columns.

### Describe alternatives you've considered

Alternatively you could allow configuring which columns to handle through metadata & ideally exclude generated ones by default.

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

I don’t believe I know code base well enough to be able to contribute a patch