fix: OPDS browser OOM (#403)

## Summary

- Rewrite OpdsParser to stream parsing instead of full content
- Fix OOM due to big http xml response

Closes #385 

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src/network/HttpDownloader.h

@@ -27,6 +27,8 @@ class HttpDownloader {
    */
   static bool fetchUrl(const std::string& url, std::string& outContent);
 
+  static bool fetchUrl(const std::string& url, Stream& stream);
+
   /**
    * Download a file to the SD card.
    * @param url The URL to download