Overridable body and headers

This PR allows the header or body to be overridden when using the consumer API to send simulated requests to a provider. The way it works is that the Actor produces a request as before (either based on an example or the JSON schema). The difference is that the caller can pass a proc to modify the body or headers before they are sent:

```ruby
interactions = Pacto.simulate_consumer :my_client do
  request 'Ping'
  request 'Echo', body: ->(body) { body.reverse },
                  headers: (proc do |headers|
                    headers['Content-Type'] = 'text/json'
                    headers['Accept'] = 'none'
                    headers
                  end)
```

This should unblock many tests that were problematic before, like tests that needed to inject or modify headers for authentication, UUIDs, or crypographic signatures.

Eventually better middleware or features may be available for common scenarios like injecting authentication information, but for now I think this is a simple workaround.
I should probably put something here...

Still, I think this makes sense even if it just ends up being for documentation.