Expose a resolver in the SPI to lookup the deploymentUrl easily from an extension

Story:

In order to initialize clients to conveniently access URLs in the deployed application
As an extension writer
I want to be able to use an SPI to calculate the deployment URL (consistent with the URLResourceProvider)

Scenarios:

Given that I need to access the deployment URL from an extension
When I invoke a public SPI method that resolves the deployment URL
: And the test case only has one deployment
Then the SPI method should return the URL to that deployment

Given that I need to access the deployment URL from an extension
When I invoke a public SPI method that resolves the deployment URL
: And the test case has multiple deployments
: And I specify a string-based deployment name as an argument
Then the SPI method should return the URL to that deployment

Rationale:

Extensions may need access to the deploymentUrl for setting up resources, such as a client. However, there is quite a bit of logic necessary to resolve the deploymentUrl properly, especially when multiple deployments are in use.

Therefore, I propose adding an SPI that exposes the logic in the URLResourceProvider. Here's a rough sketch of the SPI:

public interface DeploymentURLResolver {
    public URL resolveURL();

    public URL resolveURL(String deploymentName);
}