feat: utilize IHttpClientFactory for improved HTTP client management

- Replaced direct `HttpClient` dependencies with `IHttpClientFactory` in `GithubService`, `ForgejoService`, and `ReleaseAssetDownloadService`.
- Added DI-configured HTTP client registrations for GitHub, Forgejo, and asset downloads in `Program.cs`.
- Consolidated HTTP client setup to enhance maintainability, configurability, and resource lifecycle management.

Services/ReleaseAssetDownloadService.cs

@@ -2,12 +2,14 @@ using Microsoft.Extensions.Logging;
 
 namespace ForgejoReleaseSync.Services;
 
-public class ReleaseAssetDownloadService(HttpClient httpClient, ILogger<ReleaseAssetDownloadService> logger)
+public class ReleaseAssetDownloadService(IHttpClientFactory httpClientFactory, ILogger<ReleaseAssetDownloadService> logger)
 {
     public async Task<string> DownloadReleaseAssetAsync(Uri url, CancellationToken token = default)
     {
         try
         {
+            var httpClient = httpClientFactory.CreateClient("asset-download");
+
             var fileName = Path.GetFileName(url.AbsoluteUri);
             var filePath = Path.Combine(Path.GetTempPath(), fileName);