Allow Iam tokens to be passed from the console to the SDK

[gracebenz]

Have spoken offline with Sebastian about this -- feel free to ping me and I can link to a design doc.

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Latest commit: 40cdec4

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[google-oss-bot]

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No changes between base commit (7b585e8) and head commit (7db124c).

Test Logs

• Base (7b585e8): https://github.com/firebase/firebase-js-sdk/actions/runs/688236477
• Head (7db124c): https://github.com/firebase/firebase-js-sdk/actions/runs/698158224