Add request and response cases for Optional values

[Jictyvoo]

Is your feature request related to a problem? Please describe.
Currently I'm using the Go default values for a response in a lot of fields. But currently I need to to send a pointer for optional values, and still receive the value as the response.

Describe the solution you'd like
On the file genqlient.yaml have a new config that receives the option that will be used on optional on request, and another option on response.

Describe alternatives you've considered
On the genqlient.yaml have the following alternatives:

optional:
    request: pointer
    response: value # use value as default if empty

Alternative

request:
    optional: pointer
    use_struct_references: true
response:
    optional: value
    use_struct_references: false

[benjaminjkraft]

Seems reasonable to me! As far as I can guess it should be easy enough to implement. (Although I won't likely have time soon, so PRs extra welcome!) I like the first config option.