Support the 99designs/gqlgen GraphQL Federation v0.17.5

[richgrove]

When use the https://github.com/jensneuse/graphql-go-tools/tree/master/examples/federation, which is github.com/99designs/gqlgen v0.13.0, there is no problem to get the correct result. If the federation schema is created in v0.17.5, the graphql-go-tools gateway got the following errors:

Access Playground on: http://localhost:4000/playground
2022/05/02 08:12:52 Failed to get sdl decode response: invalid character '<' looking for beginning of value
2022/05/02 08:12:52 Failed to get sdl decode response: invalid character '<' looking for beginning of value

The same federation codes running v0.17.5.0 don't have any issue when use the apollo GraphQL nodejs gateway. Please help to update the graphql-go-tools