[Auth] Add Cordova demo app

[sam-gc]

This app is based on the main demo app JS, with the bits removed that we don't support in Cordova.

[changeset-bot]

⚠️ No Changeset found

Latest commit: d2d7bd4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (36f0c20) and head commit (cd2c704).

Test Logs

• Base (36f0c201): https://github.com/firebase/firebase-js-sdk/actions/runs/968877998
• Head (cd2c7043): https://github.com/firebase/firebase-js-sdk/actions/runs/969035983

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (36f0c20) and head commit (cd2c704).

Test Logs

• Base (36f0c20): https://github.com/firebase/firebase-js-sdk/actions/runs/968877998
• Head (cd2c704): https://github.com/firebase/firebase-js-sdk/actions/runs/969035983