Allow compatibility when using introspection derived client schema's (#145)

During generation we parse the schema using [`gqlparser.LoadSchema`](https://github.com/vektah/gqlparser/blob/2a3d320c0f1d31f404cc36f6cce8f7f93b016682/gqlparser.go#L11) over [here](https://github.com/Khan/genqlient/blob/a4aa6d9bb0f45cb71b3b7816742172011d96fbc1/generate/parse.go#L34). As you can see `gqlparser.LoadSchema` uses it's sub directory declared [`validator.LoadSchema`](https://github.com/vektah/gqlparser/blob/2a3d320c0f1d31f404cc36f6cce8f7f93b016682/gqlparser.go#L12) - but prepends a schema with [typical implicit declared types](https://github.com/vektah/gqlparser/blob/2a3d320c0f1d31f404cc36f6cce8f7f93b016682/validator/prelude.go#L5). Full server schema introspection exposes the entire schema explicitly, hence causing a clash with this prelude schema rendering introspection derived schemas to fail.

Here I just introduce a two stage schema parsing step to accomodate both implicit and explicit sdl's.

--

I have also added to the FAQ as per https://github.com/Khan/genqlient/issues/4 how we can use introspection to fetch the schema when using `genqlient`.

Co-authored-by: Ben Kraft <[email protected]>

docs/FAQ.md

@@ -96,7 +96,7 @@ bindings:
 
 Or, you can bind it to any other type, perhaps one with size-checked constructors; see the [`genqlient.yaml` documentation](genqlient.yaml) for more details.
 
-### … let me json-marshal my response objects
+### … let me json-marshal my response objects?
 
 This is supported by default!  All genqlient-generated types support both JSON-marshaling and unmarshaling, which can be useful for putting them in a cache, inspecting them by hand, using them in mocks (although this is [not recommended](#-test-my-graphql-apis)), or anything else you can do with JSON.  It's not guaranteed that marshaling a genqlient type will produce the exact GraphQL input -- we try to get as close as we can but there are some limitations around Go zero values -- but unmarshaling again should produce the value genqlient returned.  That is:
 
@@ -109,6 +109,42 @@ var respAgain MyQueryResponse
 err := json.Unmarshal(b, &resp)
 ```
 
+### … let me use introspection to fetch my client schema?
+
+This is currently not supported by default. You can however use a tool such as [gqlfetch](https://github.com/suessflorian/gqlfetch) to build your client schema using introspection and then let `genqlient` continue from there. Moreover, you can define yourself what happens when `go:generate` is run via managing your own _go runnable_ progam.
+
+For example - suppose the file `generate/main.go`;
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/Khan/genqlient/generate"
+	"github.com/suessflorian/gqlfetch"
+)
+
+func main() {
+	schema, err := gqlfetch.BuildClientSchema(context.Background(), "http://localhost:8080/query")
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+
+	if err = os.WriteFile("schema.graphql", []byte(schema), 0644); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+
+	generate.Main()
+}
+```
+
+This can now be invoked upon `go generate` via `//go:generate yourpkg/generate`.
+
 ## How do I make a query with …
 
 ### … a specific name for a field?