[AltServer] Moves core ConnectionManager logic to AltKit

Refactors ConnectionManager to use arbitrary RequestHandlers and ConnectionHandlers. This allows the core AltServer request logic to be shared across different targets with different connection types.

AltKit/Server Protocol/CodableServerError.swift

@@ -0,0 +1,64 @@
+//
+//  CodableServerError.swift
+//  AltKit
+//
+//  Created by Riley Testut on 3/5/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import Foundation
+
+// Can only automatically conform ALTServerError.Code to Codable, not ALTServerError itself
+extension ALTServerError.Code: Codable {}
+
+struct CodableServerError: Codable
+{
+    var error: ALTServerError {
+        return ALTServerError(self.errorCode, userInfo: self.userInfo ?? [:])
+    }
+    
+    private var errorCode: ALTServerError.Code
+    private var userInfo: [String: String]?
+    
+    private enum CodingKeys: String, CodingKey
+    {
+        case errorCode
+        case userInfo
+    }
+
+    init(error: ALTServerError)
+    {
+        self.errorCode = error.code
+        
+        var userInfo = error.userInfo.compactMapValues { $0 as? String }
+        
+        if let localizedRecoverySuggestion = (error as NSError).localizedRecoverySuggestion
+        {
+            userInfo[NSLocalizedRecoverySuggestionErrorKey] = localizedRecoverySuggestion
+        }
+        
+        if !userInfo.isEmpty
+        {
+            self.userInfo = userInfo
+        }
+    }
+    
+    init(from decoder: Decoder) throws
+    {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        
+        let errorCode = try container.decode(Int.self, forKey: .errorCode)
+        self.errorCode = ALTServerError.Code(rawValue: errorCode) ?? .unknown
+        
+        let userInfo = try container.decodeIfPresent([String: String].self, forKey: .userInfo)
+        self.userInfo = userInfo
+    }
+    
+    func encode(to encoder: Encoder) throws
+    {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(self.error.code.rawValue, forKey: .errorCode)
+        try container.encodeIfPresent(self.userInfo, forKey: .userInfo)
+    }
+}
+