Provide an API for listing samples

It would be useful for other tools to be able to report discovered samples and their metadata (URL, title, etc)

### Expected Behavior
sample-discovery should provide an API that lists samples and their metadata.

### Current Behavior
No such API exists

### Context
A couple Gradle projects have requested the ability to have indexed samples. Listing samples in a report is the first step toward this.

The issue names no files or tests; start by locating the sample-discovery entry point and the existing representation of sample metadata. Determine the API shape needed to list discovered samples, including fields such as URL and title. Done means other tools can retrieve a complete sample listing with its metadata.

Written by the indexing model from the issue text.