[FR] Add check to verify if auth emulator host is valid

**Is your feature request related to a problem? Please describe.**
I accidentally used `http://127.0.0.1:9099` instead of `127.0.0.1:9099` when setting `FIREBASE_AUTH_EMULATOR_HOST`.

Even though the emulator thew an error like below, it still took me a while to figure out what I did wrong 🤦 :
```
FirebaseAppError: Error while making request: getaddrinfo ENOTFOUND http. Error code: ENOTFOUND
at C:\Users\PATH\firebase-admin-node\lib\utils\api-request.js:268:19
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
at async main (file:///C:/Users/PATH/auth.js:12:5) {
errorInfo: {
code: 'app/network-error',
message: 'Error while making request: getaddrinfo ENOTFOUND http. Error code: ENOTFOUND'
},
codePrefix: 'app'
}
```

**Describe the solution you'd like**
Throw an error message indicating that `http` and `https` should not be included when setting `FIREBASE_AUTH_EMULATOR_HOST`. Found a similar logic in https://github.com/firebase/firebase-admin-node/blob/a46086b61f58f07426a6ca103e00385ae216691d/src/storage/storage.ts#L51-L56

**Describe alternatives you've considered**
N/A

**Additional context**
N/A

Search the repository for FIREBASE_AUTH_EMULATOR_HOST handling and compare it with the validation logic linked in src/storage/storage.ts. Confirm the expected host format and add the requested diagnostic for http or https prefixes, then run the relevant authentication tests to verify the invalid setting is reported clearly.

Written by the indexing model from the issue text.