Action: Enter purchase process

[mtnygard]

Write the action for :action.type/process-enter, in namespace simtest.actions.process-enter.

Must record that the agent began the checkout process. Must also issue an HTTP POST to a URL representing checkout.

For the purpose of this example, we won't be implementing a real checkout process. Posting to the URL /cart will be our toy equivalent of entering checkout.

This action must record the response time, response code, and response body.