Refactor: use a single .json file to describe all cached artifact bundles (#121)

This is a pure refactor, moving from a separate .cache file per bundle to a single cache-metadata.json file describing all bundles. Instead of storing cache metadata in a separate .cache file per artifact bundle, all of the metadata is now stored in a single `.json` file.

This will make it easier to implement more flexible artifact-caching strategies, such as caching each wrapper zip separately.

* Always include cache protocol version in cache key
* Store all cache metadata in a single JSON file
* Rename cache-metadata file and bump protocol version
* Polish and documentation

src/cache-project-dot-gradle.ts

@@ -7,6 +7,9 @@ const PATHS_TO_CACHE = [
     'configuration-cache' // Only configuration-cache is stored at present
 ]
 
+/**
+ * A simple cache that saves and restores the '.gradle/configuration-cache' directory in the project root.
+ */
 export class ProjectDotGradleCache extends AbstractCache {
     private rootDir: string
     constructor(rootDir: string) {