Make fetch implementation configurable

[ctavan]

Currently apollo-client relies on the globally available fetch api
which causes a number of problems. In the browser apollo-client uses a
polyfill which "pollutes" the global scope. This is something a library
should probably rather not do.

This change is based on previous discussion and work done in
#661 and #645.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

Comments:

I believe that the current state where apollo-client inserts a fetch-polyfill and thus modifies the global scope is a behavior that a library should not have. I would like to use apollo-client within another library myself and I do not want to pollute the globale scope of they sites who will use my library.

The most straightforward change I can think of is allowing to optionally pass a fetch implementation as configuration option when creating the network interface.

This PR still has some rough edges. My main concerns are:

• Is fetch a good variable name for the config option?
• The fetch option is currently defined as type any which should of course be changed.
• The fetch-ponyfill type definitions could/should be improved, they are now just very generic.
• Documentation is missing.
• Changelog.

I'm curious about your feedback and I'll be happy fix the open points in case there's a chance to get this merged.

More references: #1134, #1155

[apollo-cla]

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[mention-bot]

@ctavan, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jbaxleyiii, @helfer and @Poincare to be potential reviewers.

[apollo-cla]

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[ctavan]

Oh nose, I just discovered #1993 and https://github.com/apollographql/apollo-fetch which will make this one irrelevant…

I guess I'll take a closer look at apollo-fetch then as it seems that this also still globally imports isomorphic-fetch…