feat: add api_key credentials

[arithmetic1728]

This PR adds a new credential type for API key, and the get_api_key_credentials method needed by GAPIC clients (see here) to enable API key support. Once this PR is released in auth lib, GAPIC clients will be able to use API key with the latest auth lib. This feature has been tested with the following sample.

import google.cloud.language_v1
import google.api_core.client_options

# provide api key value via client options
client = google.cloud.language_v1.LanguageServiceClient(client_options={"api_key":"FILL IN"})

request = {'document': { 'type_': 'PLAIN_TEXT', 'content': "hello" }, 'encoding_type': 'UTF8'}
response = client.analyze_sentiment(request)

print(response)