Refactor for performance and add documentation

This commit completely refactors the CheckProxy application to be more performant, efficient, and user-friendly.

- Replaced synchronous network calls with asynchronous operations throughout the application.
- Implemented concurrent proxy checking using `Task.WhenAll` and a `SemaphoreSlim` to control parallelism, significantly speeding up batch processing from a file.
- Replaced basic argument parsing with the `System.CommandLine` library, providing a robust CLI with support for single proxy checks, file inputs (`--file`), and configurable timeouts (`--timeout`).
- Consolidated six redundant proxy check methods into three focused and reliable checks.
- Removed the unused `RestSharp` dependency in favor of the modern `HttpClient`.
- Added comprehensive XML documentation to the C# code for better maintainability.
- Overhauled the `README.md` to provide accurate and detailed information, including build instructions and up-to-date usage examples.

Author: google-labs-jules[bot]

CheckProxy.csproj

@@ -13,8 +13,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="RestSharp" Version="108.0.3" />
     <PackageReference Include="Spectre.Console" Version="0.45.0" />
+    <PackageReference Include="System.CommandLine" Version="2.0.0-beta4.22272.1" />
   </ItemGroup>
 
 </Project>