Add output caching for Forge 1.13+.

CHANGES
 - Forge installer is no longer stored to the work directory. It is now stored in a cache folder corresponding to the artifact version.
 - Installer output is now cached by default
 - Added options to generate distro.
   - --discardOutput Delete cached output after it is no longer required. May be useful if disk space is limited.
   - --invalidateCache Invalidate and delete existing caches as they are encountered. Requires fresh cache generation.
   - Both options are false by default.
 - To invalide a single version, manually delete the folder.
 - Old functionality is essentially g distro --discardOutput --invalidateCache.

src/structure/spec_model/Server.struct.ts

@@ -18,7 +18,9 @@ export class ServerStructure extends BaseModelStructure<Server> {
 
     constructor(
         absoluteRoot: string,
-        baseUrl: string
+        baseUrl: string,
+        private discardOutput: boolean,
+        private invalidateCache: boolean
     ) {
         super(absoluteRoot, '', 'servers', baseUrl)
     }
@@ -133,7 +135,9 @@ export class ServerStructure extends BaseModelStructure<Server> {
                         serverMeta.forge.version,
                         dirname(this.containerDirectory),
                         '',
-                        this.baseUrl
+                        this.baseUrl,
+                        this.discardOutput,
+                        this.invalidateCache
                     )
 
                     // Resolve forge