go schemabuilder/input: Add ability to have optional input fields.
go schemabuilder/input: Add ability to have optional input fields.

Summary: Updates the input parameter arg parser for structs to support
having fields which are "optional".  If these fields are not provided to
the graphql functions by the query, they will get filled with the
default values of that type (nothing).

samsarahq/thunder,samsarahq/thunder,samsarahq/thunder,samsarahq/thunder,samsarahq/thunder