[FR] Support Edge and Deno runtime

**Is your feature request related to a problem? Please describe.**

https://nextjs.org/docs/api-reference/edge-runtime

I'd like to use `firebase-admin` in an Edge environment and Deno. The package currently uses Node-specific APIs though, making that impossible.

**Describe the solution you'd like**

Would be awesome to support the Edge/Deno runtime, e.g. via `firebase-admin/common`.

Start with the linked Edge Runtime documentation and review how firebase-admin currently depends on Node-specific APIs. Identify the scope of support needed for Edge and Deno, including the proposed firebase-admin/common entry point, and define completion as usable package support in both runtimes.

索引モデルが issue の本文から書いたものです。