title: Associate a usage plan to api key using boto3 AWS api gateway
body: <p>Hey I am using boto3 library to create a new api key. I successfully created a new api key but I am having trouble associate it with the existing usage plan. Can you please help me to associate api-key to existing usage plan
  My code to create api key</p>
  <pre><code>import boto3
  apiclient= boto3.client('apigateway')
  response= apiclient.create_api_key(
          name='string',
          enabled=True,
          stageKeys=[{
              'restApiId':'string',
              'stageName':'string'
          }]
          )
  </code></pre>