Allow endpoint-specific field normalization/denormalization

[NatPri]

Allows clients to specify field name normalization/denormalizations on a per-endpoint basis. This would be useful in cases where (for instance) one endpoint picks PascalCase for their field names and another picks camelCase, and you want to standardize on one or the other in your code.

This could be worked-around using a responseTransformer and per-request denormalizers, but that feels pretty heavy for this specific type of transformation.

Implements the feature described in #232

[apollo-cla]

@NatPri: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[codecov-io]

Codecov Report

Merging #233 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #233   +/-   ##
=======================================
  Coverage   92.71%   92.71%           
=======================================
  Files           3        3           
  Lines         412      412           
  Branches      123      124    +1     
=======================================
  Hits          382      382           
  Misses         28       28           
  Partials        2        2

Impacted Files	Coverage Δ
src/restLink.ts	92.48% <100%> (ø)	⬆️

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[NatPri]

I need to figure out what approvals I need to get with my workplace to sign the CLA. I will get back to this PR once I have that sorted out.

[fbartho]

Hey @NatPri -- please let me know if you still want this feature?

To go forwards, we need to add some unit tests, and some docs + changelog about it!

[NatPri]

I'm still interested in the feature; once I have some time to handle getting the proper authorizations in place I'll definitely come back to this. If you have a policy against leaving PRs open for too long, however, I can open a new one once I've gotten all my ducks in a row.

[fbartho]

No policy @NatPri -- but I'm going to drop it from the 8.0 milestone for now! Tag me when you want me to take another look!

[fbartho]

@NatPri do you still want to refresh this PR? I'm going to close it otherwise!

[fbartho]

@NatPri -- should you get around to updating this PR, I could ship it soon!

We've added perRequest normalization/denormalization since this PR was originally so there's going to be some conflicts.

I'm going to close this PR for now, but if you want it reopened, I'm happy to do so.