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/main.ts

@@ -7,7 +7,9 @@ import * as execution from './execution'
 import * as gradlew from './gradlew'
 import * as provision from './provision'
 
-// Invoked by GitHub Actions
+/**
+ * The main entry point for the action, called by Github Actions for the step.
+ */
 export async function run(): Promise<void> {
     try {
         const workspaceDirectory = process.env[`GITHUB_WORKSPACE`] || ''