Add cache-provider: external to skip Gradle User Home caching (#1059)

Users relying on an external mechanism to save/restore Gradle User Home
(e.g. Develocity Artifact Cache) previously had to set cache-disabled:
true, which is confusing since caching isn't actually disabled — it's
just not managed by this action — and the Job Summary misleadingly
reported caching as "Disabled".

cache-provider: external skips Gradle User Home restore/save (same as
cache-disabled) but reports a distinct "External" status in the Job
Summary, explaining that caching is handled by another provider.

---------

Co-authored-by: Claude Sonnet 5 <[email protected]>

sources/src/configuration.ts

@@ -175,14 +175,19 @@ export class CacheConfig {
             case 'enhanced':
             case '':
                 return CacheProvider.Enhanced
+            case 'external':
+                return CacheProvider.External
         }
-        throw TypeError(`The value '${val}' is not valid for 'cache-provider'. Valid values are: [basic, enhanced].`)
+        throw TypeError(
+            `The value '${val}' is not valid for 'cache-provider'. Valid values are: [basic, enhanced, external].`
+        )
     }
 }
 
 export enum CacheProvider {
     Basic = 'basic',
-    Enhanced = 'enhanced'
+    Enhanced = 'enhanced',
+    External = 'external'
 }
 
 export enum CacheCleanupOption {