feat(firestore): add toJSON() and valueOf() to FirestoreTimestamp

[pokey]

Description

For parity with JS SDK, and also to make comparisons work as expected. Copied from https://github.com/firebase/firebase-js-sdk/blob/7c1c7f182b59e0fc7d175f53e5e2360cdee0ccab/packages/firestore/src/api/timestamp.ts#L162-L182.

I just copied and pasted without testing or modification, so please lmk if this is something you'd be interested in including, and I'd be happy to add a unit test / clean / add type info to proper place

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

---

Think react-native-firebase is great? Please consider supporting the project with any of the below:

• 👉 Star this repo on GitHub ⭐️
• 👉 Follow React Native Firebase and Invertase on Twitter

[CLAassistant]

All committers have signed the CLA.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/invertase/react-native-firebase/a1v1mzwb1
✅ Preview: https://react-native-firebase-git-patch-1.invertase.vercel.app

[codecov]

Codecov Report

Merging #4439 (98a13b9) into master (513c345) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4439   +/-   ##
=======================================
  Coverage   41.92%   41.92%           
=======================================
  Files          35       35           
  Lines        1119     1119           
  Branches      278      278           
=======================================
  Hits          469      469           
  Misses        489      489           
  Partials      161      161           

[mikehardy]

@pokey 🏆 ! Sorry for the delay in reviewing this, there was one small lint error I corrected, but otherwise this looked great, I will merge this and do a release in an hour or so

[davidstott]

Should FirestoreTimestamp include MIN_SECONDS for valueOf to work?
Something like: const MIN_SECONDS = -62135596800;

[mikehardy]

@davidstott great catch! That's what we get for using javascript still instead of typescript 😞

The relevant lines we are missing are these https://github.com/firebase/firebase-js-sdk/blob/7c1c7f182b59e0fc7d175f53e5e2360cdee0ccab/packages/firestore/src/api/timestamp.ts#L21-L22

// The earlist date supported by Firestore timestamps (0001-01-01T00:00:00Z).
const MIN_SECONDS = -62135596800;

I'll PR it quickly