Support optional: generic inline

[benjaminjkraft]

Is your feature request related to a problem? Please describe.
We now have support for mapping GraphQL optional types to generics (see #251). In many schemas, though, you don't want to deal with that everywhere; for example for some fields mapping to empty-string is clear enough and easier to use. We should allow flagging the generic for just certain fields.

Describe the solution you'd like
Probably the way to do this is to still put optional_generic_type in the genqlient.yaml, but omit optional: generic and then do that inline. We could also let you specify both inline, but I really hope you don't need that and requiring it would be very verbose.

Describe alternatives you've considered
As anyone who has used genqlient knows, I hate using pointers for optionality! But sometimes zero values don't cut it.