Add support for user-provided Google Books API key to fix "429 Client Error: Too Many Requests"

[cjwin2000]

Using the Google Books API without a key is limited to a hard cap of global daily requests. Once the quota is reached, all requests without a key return a 429 error. This can be worked around by using a free personal API key generated in the Google Cloud console. The key allows for 1000 requests a day, and users can request a limit increase for free from Google if that is not enough. API key can be added in Admin Panel -> Basic Settings -> Feature Configuration.

Functionality for users that do not wish to use a personal API key remains unchanged.

I don't see an open issue, but here is a discord user running into it. Here is the json response from that request when the quota is reached:

{
  "error": {
    "code": 429,
    "message": "Quota exceeded for quota metric 'Queries' and limit 'Queries per day' of service 'books.googleapis.com' for consumer 'project_number:624717413613'.",
    "errors": [
      {
        "message": "Quota exceeded for quota metric 'Queries' and limit 'Queries per day' of service 'books.googleapis.com' for consumer 'project_number:624717413613'.",
        "domain": "global",
        "reason": "rateLimitExceeded"
      }
    ],
    "status": "RESOURCE_EXHAUSTED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "RATE_LIMIT_EXCEEDED",
        "domain": "googleapis.com",
        "metadata": {
          "quota_location": "global",
          "quota_limit_value": "20000000",
          "consumer": "projects/624717413613",
          "quota_limit": "defaultPerDayPerProject",
          "service": "books.googleapis.com",
          "quota_metric": "books.googleapis.com/default"
        }
      },
      {
        "@type": "type.googleapis.com/google.rpc.Help",
        "links": [
          {
            "description": "Request a higher quota limit.",
            "url": "https://cloud.google.com/docs/quotas/help/request_increase"
          }
        ]
      }
    ]
  }
}