feat: support for API Key client option

[bshaffer]

Adds support for API Keys in GAPIC clients via a new apiKey client option:

$productSearchClient = new ProductSearchClient([
    'apiKey' => $myApiKey
]);

AIP: https://google.aip.dev/auth/4110

• Supports the apiKey client option - this will be sent in using the x-goog-api-key header for all transports.
• If the apiKey and credentials or credentialsConfig.keyFile option are supplied, an exception is thrown
• If the apiKey option AND the quotaProject option are set, the $quotaProject is still sent in