Add externs annotation to firebase-externs

[hsubox76]

An internal change was requested (see cl/356656077), this makes it permanent and prevents that change from being overwritten next time the firebase library is imported to google3.

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Latest commit: 6587d12

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (318af54) and head commit (b0d8210).

Test Logs

• Base (318af54): https://github.com/firebase/firebase-js-sdk/actions/runs/553271170
• Head (b0d8210): https://github.com/firebase/firebase-js-sdk/actions/runs/555548452

[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (318af54) and head commit (b0d8210).

Test Logs

• Base (318af547): https://github.com/firebase/firebase-js-sdk/actions/runs/555538614
• Head (b0d8210e): https://github.com/firebase/firebase-js-sdk/actions/runs/555548452