Federation docs and examples

[vektah]

Adds an example implementing the federation introduction server, a short recipe on how it was created and an end to end test to make sure federation doesn't get completely broken.

@marwan-at-work

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[marwan-at-work]

This is great 👍

As a side note, it might be worth throwing a go.mod file in the example folder (or making a separate repo since sub-modules come with a big caution) so that someone go-getting gqlgen doesn't need to download the entire examples folder.

[vektah]

I don't want to split the examples out into their own repo because its really hard to keep external resources in sync. Look how well it turned out for https://github.com/vektah/gqlgen-tutorials :(

In this case though, the go mod changes aren't coming from the example, but from a bit of debugging middleware I needed to debug an issue.

side note: Federation gateway is pretty dumb hey?

That said, these new deps are pretty tiny and shouldn't cause any issues. Its a shame go mod tidy doesn't prune test/unused deps itself. At least go doesn't build these packages, and the downloads are fast (via proxy)