Add default Apollo Engine endpoint

[lsanwick]

Generally, users are not going to provide a custom endpoint for fetching from the Apollo Engine, so make it optional and add a sensible default value.

TODO:

• If this PR is a new feature, 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
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[ardatan]

Thank you @lsanwick !

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.0.4-alpha-68c14f5.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.0.4-alpha-68c14f5.0