`Code generated by` disclaimer should be at the top of the file, as it is demonstrated in [this article](https://go.dev/blog/generate) on The Go Blog.
The current placing breaks integration with some code formatters and linters which do no skip files generated by `genqlient`.
Adds a "struct_references" configuration that will:
* Use a pointer type for struct fields that are a complex type
* The behaviour can be overridden by setting "pointer: false" in the
proceeding comment blocks
* Sets the "omitempty: true" flag on fields matching the criteria.
This can also be overriden by setting "omitempty: false"
Although we'd debated not setting the pointer for array elements, it did turn out to be simpler to set them everywhere and also made the documentation cleaner to have a single rule to explain.
Fixes#149.
