chore: Don't set keyFile when credentialsFetcher is given

[jeromegamez]

#8617 marked keyFile and keyFilePath options as deprecated.

When initializing, for example, Google\Cloud\Storage\StorageClient, with

<?php

use Google\Auth\ApplicationDefaultCredentials;
use Google\Cloud\Storage\StorageClient;

putenv('GOOGLE_APPLICATION_CREDENTIALS=/path/to/credentials.json');
$credentialsFetcher = new ApplicationDefaultCredentials([], '/path/to/other/credentials.json');

$client = new StorageClient(['credentialsFetcher' => $credentialsFetcher]);

the ClientTrait will still look for a keyFile or keyFilePath option and if it doesn't find one, fall back to checking the environment variable or a well known file, and then set the keyFile option with it.

With the changes of this PR, the ClientTrait will check if the credentialsFetcher is set and implements FetchAuthTokenInterface, and if it is, it will not look for or set the keyFile option. When determining the project ID, it will also check if the credentialsFetcher implements the ProjectIdProviderInterface to determine the project it.

If no credentialsFetcher is given, it will fall back to the currently existing behavior.