function deploy_subscribe(fn) {
	listener = fn
}
Used externally to listen for deployments & perform the deployment.

@async
@param {function} fn - The function to be invoked during deployment.
@param {DeploymentPayload} fn.payload - The site bundle & destination repo.
@param {boolean} fn.create_new - Flag indicating whether a new repository should be created if it doesn't exist.
@returns {Promise<{DeploymentResponse}>} - A Promise that resolves to an object containing details of the deployment.