This PR fixes #1836.

This PR enables developers to inject the http agent to be used on the network requests to apollo engine endpoint.

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TODO:

* [x] Update CHANGELOG.md with your change (include reference to issue & this PR)
* [x] Make sure all of the significant new logic is covered by tests
* [x] Rebase your changes on master so that they can be merged easily
* [x] Make sure all tests and linter rules pass

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