[AltServer] Installs Developer disk image before installing AltStore

Allows AltServer to programmatically initiate a debug session with AltStore, which can be used to start a background refresh or enable JIT on demand.

[AltServer] Renames ALTDevice variable name

AltServer/Extensions/FileManager+URLs.swift

@@ -0,0 +1,29 @@
+//
+//  FileManager+URLs.swift
+//  AltServer
+//
+//  Created by Riley Testut on 2/23/21.
+//  Copyright © 2021 Riley Testut. All rights reserved.
+//
+
+import Foundation
+
+extension FileManager
+{
+    var altserverDirectory: URL {
+        let applicationSupportDirectoryURL = self.urls(for: .applicationSupportDirectory, in: .userDomainMask)[0]
+        
+        let altserverDirectoryURL = applicationSupportDirectoryURL.appendingPathComponent("com.rileytestut.AltServer")
+        return altserverDirectoryURL
+    }
+    
+    var certificatesDirectory: URL {
+        let certificatesDirectoryURL = self.altserverDirectory.appendingPathComponent("Certificates")
+        return certificatesDirectoryURL
+    }
+    
+    var developerDisksDirectory: URL {
+        let developerDisksDirectoryURL = self.altserverDirectory.appendingPathComponent("DeveloperDiskImages")
+        return developerDisksDirectoryURL
+    }
+}