Refactor Shorebird CLI to support configurable URLs and improve test coverage

- Updated AAR releaser test to use ShorebirdProcess.defaultFlutterStorageBaseUrl.
- Changed macOS releaser test to reference a new troubleshooting URL.
- Enhanced shorebird_yaml_test with deserialization for AOT patch metadata.
- Modified network_checker_test to check self-hosted URLs from ShorebirdEnv.
- Added tests for artifact paths in shorebird_artifacts_test.
- Updated shorebird_cli_command_runner_test to reflect new repository URL.
- Improved shorebird_env_test with additional tests for shorebirdRoot and engine revision fallback.
- Adjusted shorebird_flutter_test to use environment variables for Flutter Git URL.
- Updated shorebird_process_test to utilize default Flutter storage URL.
- Enhanced shorebird_validator_test to link to the default hosted URL.
- Updated shorebird_web_console_test to use the configured hosted URL.
- Refactored code_push_client to use default hosted URL for API requests.
- Updated README and generation scripts for the code_push_protocol package to reflect new OpenAPI spec source.
- Modified shared.sh to allow configurable Flutter repository and storage URLs.

Signed-off-by: Tony <tonylu@tony-cloud.com>

bin/shorebird.ps1

@@ -15,6 +15,8 @@ $flutterPath = [IO.Path]::Combine($shorebirdCacheDir, "flutter", $flutterVersion
 $flutter = [IO.Path]::Combine($shorebirdCacheDir, "flutter", $flutterVersion, "bin", "flutter.bat")
 $shorebirdScript = [IO.Path]::Combine($shorebirdCliDir, "bin", "shorebird.dart")
 $dart = [IO.Path]::Combine($flutterPath, "bin", "cache", "dart-sdk", "bin", "dart.exe")
+$defaultFlutterGitUrl = "https://git.tonycloud.org/flutter/flutter.git"
+$defaultFlutterStorageBaseUrl = "http://localhost:8080/download.flutter.io"
 
 # Executes $command and redirects as much output to $null as possible.
 #
@@ -105,10 +107,21 @@ function Test-ShorebirdNeedsUpdate {
 
 function Update-Flutter {
     Write-Output "Updating Flutter..."
+    $flutterGitUrl = $env:SHOREBIRD_FLUTTER_GIT_URL
+    if ([string]::IsNullOrWhiteSpace($flutterGitUrl)) {
+        $flutterGitUrl = $defaultFlutterGitUrl
+    }
+    $flutterStorageBaseUrl = $env:SHOREBIRD_FLUTTER_STORAGE_BASE_URL
+    if ([string]::IsNullOrWhiteSpace($flutterStorageBaseUrl)) {
+        $flutterStorageBaseUrl = $env:FLUTTER_STORAGE_BASE_URL
+    }
+    if ([string]::IsNullOrWhiteSpace($flutterStorageBaseUrl)) {
+        $flutterStorageBaseUrl = $defaultFlutterStorageBaseUrl
+    }
 
     if (!(Test-Path $flutterPath)) {
         Invoke-SilentlyIfNeeded {
-            git clone --filter=tree:0 https://github.com/shorebirdtech/flutter.git --no-checkout "$flutterPath" 
+            git clone --filter=tree:0 "$flutterGitUrl" --no-checkout "$flutterPath"
         }
     }
     else {
@@ -122,11 +135,18 @@ function Update-Flutter {
         git -C "$flutterPath" -c advice.detachedHead=false checkout "$flutterVersion"
     }
 
-    # Set FLUTTER_STORAGE_BASE_URL=https://download.shorebird.dev and execute
-    # a `flutter` command to trigger a download of Dart, etc.
-    $env:FLUTTER_STORAGE_BASE_URL = 'https://download.shorebird.dev';
+    # Set FLUTTER_STORAGE_BASE_URL and execute a `flutter` command to trigger
+    # a download of Dart, etc. through the configured open mirror.
+    $hadFlutterStorageBaseUrl = Test-Path Env:\FLUTTER_STORAGE_BASE_URL
+    $previousFlutterStorageBaseUrl = $env:FLUTTER_STORAGE_BASE_URL
+    $env:FLUTTER_STORAGE_BASE_URL = $flutterStorageBaseUrl;
     & $flutter --version
-    Remove-Item Env:\FLUTTER_STORAGE_BASE_URL
+    if ($hadFlutterStorageBaseUrl) {
+        $env:FLUTTER_STORAGE_BASE_URL = $previousFlutterStorageBaseUrl
+    }
+    else {
+        Remove-Item Env:\FLUTTER_STORAGE_BASE_URL -ErrorAction SilentlyContinue
+    }
 }
 
 function Update-Shorebird {