Add ability to chain rest links

[willyboy]

POC PR for chaining Rest Links

If a Rest Link directive has an endpoint defined but the current Rest Link does not have a matching endpoint registered, don't remove that directive. Assume it will be defined later down the link chain.

Caveats:

1. Only endpoints can be used because that's how we know what directives to keep/remove. We could figure out another toggle/option to control this. If a URI was used, everything with an undefined endpoint would default to it and the feature wouldn't work.
2. This is POC code. I didn't add any tests, docs, etc.
3. This code doesn't currently protect against an endpoint being undefined once getting to the end of the Link chain

Issue reference: #308

[apollo-cla]

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[fbartho]

Conceptually I like the approach!

I think I'd like an option-flag that declares enableRestLinkChaining (or isTerminalLink + default: true), that way this doesn't have to be a breaking change for the whole link, but rather just becomes a feature release.

Any thoughts?

[willyboy]

I'm leaning towards enableChaining. I like the feature flag nature of it so that if chaining became the default, it could be more easily deprecated.

[willyboy]

Made some updates. If this works I can go ahead and add a unit test and an example in the docs.