Expose Relay schema extensions (#3859)
Expose Relay schema extensions (#3859)

Summary:
fixes https://github.com/facebook/relay/issues/3787

# Context

From erictaylor:

> Pre v13 I know that `relay-compiler` exposed some of the schema extensions that are specific to Relay (ie directives) so that other third party libraries and tools could easily add them (ex: IDE GraphQL hinting/tooling, etc).
>
> I noticed that since v13 that `relay-compiler` has been simplified (obviously) down to just exposing the underlying binary.

From josephsavona

> It seems reasonable to expose the schema extensions for other tools to pick up, we'd be happy to support a PR to that effect.

# Test

Run `RELEASE_COMMIT_SHA=212d8d66d9031180f08acf76f43c88162c6208e0 yarn gulp mainrelease`

## Before
<img width="360" alt="Screen Shot 2022-04-04 at 5 42 54 PM" src="https://user-images.githubusercontent.com/127199/161656917-f9407b8d-2760-4230-bdf7-7378a82625b7.png">

## After
<img width="261" alt="Screen Shot 2022-04-04 at 5 43 40 PM" src="https://user-images.githubusercontent.com/127199/161656926-5807a58f-6016-4638-955f-39870a5e0bc6.png">

Pull Request resolved: https://github.com/facebook/relay/pull/3859

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

**Static Docs Preview: relay**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D35385475/V2/relay/)|

|**Modified Pages**|

Reviewed By: alunyov

Differential Revision: D35385475

Pulled By: captbaritone

fbshipit-source-id: a660112b7e9604d6b32d2066fdce862f0c96a294

kassens/relay,xuorig/relay,voideanvalue/relay,xuorig/relay,kassens/relay,wincent/relay,wincent/relay,kassens/relay,wincent/relay,facebook/relay,facebook/relay,facebook/relay,wincent/relay,voideanvalue/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,facebook/relay,facebook/relay,wincent/relay,voideanvalue/relay,kassens/relay,xuorig/relay,voideanvalue/relay,kassens/relay,kassens/relay,wincent/relay,facebook/relay,xuorig/relay,xuorig/relay