Make gqlgen StructFieldsAlwaysPointers configurable

[mlaflamm]

When updating to a more recent version of gqlgenc, we encountered issues with the generation of models for non-null fields. Non-null structs fields are always pointers instead of being defined as values. We were impacted by the following two changes:

• Match gqlgen default settings: Match gqlgen default settings #202
• struct type field for fragment is pointer: 2bb5b94. This one is quite old, but that change was in only done in client v2, and we were still using client v1.

This PR is to make the generation of non-null structs fields as pointers configurable.

I've used the same name used by gqlgen for the similar config: struct_fields_always_pointers. Unfortunately, that name doesn't fit well with other settings like clientInterfaceName and omitEmptyTypes. Do you prefer a different name? Perhaps structFieldsAlwaysPointers?

[Yamashou]

@mlaflamm Thanks! Let's name it structFieldsAlwaysPointers!