Initial FDC resolver support in functions SDK. (#1784)

* Initial FDC resolver support in functions SDK.

* Some fixes.

* Some more fixes.

* Add dependency on `graphql`.

* Use Apollo Server v5.

* Fix function to return not a Promise.

* Try to fix package-lock.json.

* Fix some of the error catching.

* Move some errors out of try-catch block.

* Add unit tests.

* Have `onGraphRequest` set a `dataConnectHttpsTrigger` instead of an `httpsTrigger`.

* Don't wrap around onRequest.

* Address review comments.

* Add schemaFilePath information to trigger.

* Address review comments.

* A couple docstring updates.

Author: rosalyntan

CHANGELOG.md

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+- Add `onGraphRequest()` function for handling GraphQL HTTPS requests for Firebase Data Connect. (#1784)