Add CORS header to api.flutter.dev for docs.flutter.dev

[schultek]

We are adding a new feature to docs.flutter.dev to show an interactive tooltip when hovering over API docs links. For this we need to fetch the api docs on the client, which is currently blocked by a missing CORS header on api.flutter.dev.

Concept:

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We should consider adding the missing CORS header to the firebase.json file.

Feature PR here: flutter/website#12600

CC @redbrogdon since you did the last edit in the firebase.json file.
CC @parlough