chore: Refactor Auth Token Middleware's token logic

[yash30201]

Delegate Auth Metadata logic to Auth Library

Implement the go/php-auth-token-unifomity design to delegate token fetching and metadata updation logic to the auth library, resulting transfer of auth metadata control to auth library, thus unifying the token flow.

Post this change, all the php api client services libraries would adhere to go/php-auth-token-unifomity.

[bshaffer]

The way you have it now seems to be making more changes than necessary. If the goal of this PR is to just to use updateMetadata when available, I think it would be better to simplify this logic as something like this:

// Use "updateMetadata" if it exists, otherwise use "fetchAuthToken"
if (!$this->fetcher instanceof UpdateMetadataInterface ||
   ($this->fetcher FetchAuthTokenCache && !$this->fetcher->getFetcher() instanceof UpdateMetadataInterface)
) {
    // previous behavior (unlikely - only happens if user supplies a custom fetcher)
    $request = $request->withHeader('authorization', 'Bearer ' . $this->fetchToken());
} else {
    // new behavior - allow the credentials to add / update request headers
    $updatedHeaders = $this->fetcher->updateMetadata($request->getHeaders(), null, $this->httpHandler);
    $request = Utils::modifyRequest($request, ['set_headers' => $updatedHeaders]);
}

[yash30201]

The way you have it now seems to be making more changes than necessary. If the goal of this PR is to just to use updateMetadata when available, I think it would be better to simplify this logic as something like this:

// Use "updateMetadata" if it exists, otherwise use "fetchAuthToken"
if (!$this->fetcher instanceof UpdateMetadataInterface ||
   ($this->fetcher FetchAuthTokenCache && !$this->fetcher->getFetcher() instanceof UpdateMetadataInterface)
) {
    // previous behavior (unlikely - only happens if user supplies a custom fetcher)
    $request = $request->withHeader('authorization', 'Bearer ' . $this->fetchToken());
} else {
    // new behavior - allow the credentials to add / update request headers
    $updatedHeaders = $this->fetcher->updateMetadata($request->getHeaders(), null, $this->httpHandler);
    $request = Utils::modifyRequest($request, ['set_headers' => $updatedHeaders]);
}

Thanks for the pointer. We would be required to fetch auth token when using update metadata because we need to call the token callback in the event it isn't an identity token request. I've amended the logic as such, please take a look. Now it looks cleaner.

[bshaffer]

Good stuff! We're getting closer here.

[bshaffer]

This is looking great! I only have some suggestions on how to improve the tests.