Implement the Petfinder API v2 OAuth2 client-credentials authentication flow for server-to-server access

Steps

1. Register an application at petfinder.com/developers to obtain a client_id and client_secret
2. POST to api.petfinder.com/v2/oauth2/token with grant_type=client_credentials and the client_id and client_secret
3. Parse the response for token_type, expires_in of 3600 seconds, and access_token
4. Attach the token as an Authorization Bearer header on every subsequent v2 API call
5. Detect 401 responses and automatically request a fresh token when the current one expires