Add ReactNative build back, fix base64 serialization

[schmidt-sebastian]

Manually tested on iOS:

• Mutation can be written
• Document listeners work
• A Firestore Blob that is written in RN can be read from the native iOS SDK and contains the same value

Fixes: #2667

[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/firestore

  Type	Base (f54abc2)	Head (122f72c)	Diff
  browser	248 kB	248 kB	+78 B (+0.0%)
  esm2017	194 kB	194 kB	+78 B (+0.0%)
  module	246 kB	246 kB	+78 B (+0.0%)
  react-native	?	194 kB	? (?)

• @firebase/firestore/memory

  Type	Base (f54abc2)	Head (122f72c)	Diff
  browser	186 kB	186 kB	+108 B (+0.1%)
  esm2017	145 kB	145 kB	+108 B (+0.1%)
  module	184 kB	184 kB	+108 B (+0.1%)
  react-native	?	145 kB	? (?)

• firebase

  Type	Base (f54abc2)	Head (122f72c)	Diff
  firebase-firestore.js	286 kB	287 kB	+78 B (+0.0%)
  firebase-firestore.memory.js	226 kB	226 kB	+108 B (+0.0%)
  firebase.js	820 kB	820 kB	+78 B (+0.0%)

Test Logs

• Base (f54abc25): https://github.com/firebase/firebase-js-sdk/actions/runs/152278562
• Head (122f72c7): https://github.com/firebase/firebase-js-sdk/actions/runs/153047842

[schmidt-sebastian]

We should fix the ESM 2017 build before proceeding with this change (Done)

[schmidt-sebastian]

@Feiyang1 Do you have time to look at this?

[changeset-bot]

🦋 Changeset is good to go

Latest commit: f64a733

We got this.

This PR includes changesets to release 9 packages

Name	Type
firebase	Minor
@firebase/firestore	Minor
rxfire	Major
@firebase/testing	Patch
firebase-browserify-test	Patch
firebase-package-typings-test	Patch
firebase-messaging-selenium-test	Patch
firebase-typescript-test	Patch
firebase-webpack-test	Patch

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