[core] Allow overriding map metaserver URL and reset downloader cache

- Add custom meta server API.
- Implement override.
- Add reset hook and implement reset.
- Expose reset from Storage.
- Let JNI set URL and reset cache.

Signed-off-by: NoelClick <dev@noel.click>

libs/platform/platform.hpp

@@ -268,6 +268,9 @@ public:
   /// @return integer version in yyMMdd format.
   int32_t IntVersion() const;
 
+  /// Set custom meta server url to allow user-defined download servers
+  void SetCustomMetaServerUrl(std::string const & url);
+
   /// @return url for clients to download maps
   std::string MetaServerUrl() const;