Credentials class for calling Google APIs using an API key.
Uses an API key directly in the request metadata to provide authorization.
Note: ApiKeyCredentials extends from base Credentials class rather than
GoogleCredentials/OAuth2Credentials, as it does not provide an access token and is not considered
an OAuth2 credential.
Credentials credentials = ApiKeyCredentials.create("your api key");
The callback that receives the result of the asynchronous Credentials#getRequestMetadata(java.net.URI, java.util.concurrent.Executor,
RequestMetadataCallback). Exactly one method should be called.
Interface for a service account signer. A signer for a service account is capable of signing
bytes using the private key associated with its service account.
