[Emulator] Support generateSignedPostPolicyV4 in Storage Emulator

[7hokerz]

Description:
Currently, the Firebase Storage Emulator does not fully support generateSignedPostPolicyV4.
When using this method in a local environment, it fails to route requests to the local emulator endpoint,
making it difficult to test authenticated direct uploads (POST) without hitting production buckets.

Example:

const [signedUrl] = await bucket.file(filePath).generateSignedPostPolicyV4({
    expires: expirationTime,
    conditions,
    fields,
});

I would like to propose adding support for generateSignedPostPolicyV4 within the Firebase Storage Emulator.
I have already implemented this feature and verified its functionality in a local environment.

Important Dependency Note:
This feature requires @google-cloud/storage version 7.20.0 or higher.
Previously (in v7.19.0 and below), there was an issue where STORAGE_EMULATOR_HOST was not correctly respected for certain signed URL operations.
I have personally contributed to the @google-cloud/storage library to fix this, and the fix was officially released in version 7.20.0.
Therefore, updating this dependency is a prerequisite for this feature.

Issue link
PR link

I almost have the implementation ready and would like to submit a Pull Request to contribute this feature to firebase-tools.
Please let me know if there are any specific guidelines I should follow before opening the PR.

[aalej]

Hey @7hokerz, thanks for opening up this report and for taking the initiative on working on the feature! For contribution guidelines, you can read through our CONTRIBUTING.md guide. Feel free to submit a PR whenever you're ready.