Better handling of Firestore References

[IchordeDionysos]

Instead of indexing Firestore references as:

{
  _firestore: {
    projectId: "nationalparks",
  },
  _path: {
    segments: ["nationalparks", "123"],
  },
  _converter: {},
}

they will now be indexed as:

nationalparks/123

[cla-checker-service[bot]]

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[IchordeDionysos]

Signed

[IchordeDionysos]

@JasonStoltz @byronhulcher @dackers86 @aznick this PR solves a major blocker for us. Any chance to get it approved and deployed?