1. A computer-implemented method for constructing a resource stack, comprising: under the control of one or more computer systems of a computing resource provider and configured with executable instructions, receiving, by the computing resource provider, a request to construct a resource stack defined by a configuration template that specifies at least: a computing resource selected from a plurality of computing resources offered by the computing resource provider; a custom computing resource offered by a second provider; a definition for integrating the computing resource and the custom computing resource into the resource stack; constructing the resource stack by at least: provisioning the computing resource based at least in part on the configuration template; sending a notification to the second provider instructing the second provider to configure the custom resource for use in the resource stack; receiving a response to the notification from the second provider comprising an indication of successful configuration of the custom resource; and enabling the constructed resource stack for use.