entgql: support optional description for mutation inputs

[tankbusta]

This enables support for the customization of the comment on mutation inputs auto-generated by ent.

func (TodoItem) Annotations() []schema.Annotation {
    return []schema.Annotation{
        entgql.Mutations(
            entgql.MutationCreate().
                Description("The fields used in the creation of a TodoItem"),
            entgql.MutationUpdate().
                Description("The fields used in the update of a TodoItem"),
        ),
    }
}

Generates:

"""The fields used in the creation of a TodoItem"""
input CreateTodoItem {
	"""fields omitted"""
}
"""The fields used in the update of a TodoItem"""
input UpdateTodoItem {
	"""fields omitted"""
}

[giautm]

LGTM, 💯

[a8m]

🚀