Add a new npm package for shared helpers

[mayakoneval]

What? This PR adds a new npm package @apollo/embed-helpers that contains shared code between @apollo/sandbox and @apollo/explorer.

Why? When we need to support new protocols, like @defer, multi part subscriptions, file upload, in our clients, we need to implement everything twice, in @apollo/sandbox and @apollo/explorer.

This is step one in migrating @apollo/sandbox and @apollo/explorer to share this code.

1. make new npm package
2. install it in @apollo/sandbox and @apollo/explorer, remove redundant code!

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Latest commit: 0d7b48d

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Latest deployment of this branch, based on commit 0d7b48d:

Sandbox	Source
npm-embeddable-explorer (forked)	Configuration
npm-embeddable-sandbox	Configuration
maya-test-embed-next	Configuration
maya-test-embed-next	Configuration

[Jephuff]

It might be nice to use some of these helpers in studio as well, so we only have 1 implementation of the defer stuff?
What do you think about splitting these into 2 helper packages explorer-helpers and embed-helpers?

Or I guess we could just use embed-helpers? I'm guessing there aren't any large deps in there 🤔

[mayakoneval]

@Jephuff I like that idea! I think we could reuse constructMultipartForm and readMultipartWebStream specifically in Explorer. I'll split those out 🙇🏻

[mayakoneval]

This needs to wait on release of #235