Rewrite download utils (#681)

- eTag support (It seems Mojang's CDN and our meta/maven do not support this right now)
- Age based caching.
- Sha1 based caching.
- HTTP(S) proxy settings should now be respected.
- The downloader has better awareness of offline mode and refresh deps, cleaning up the calling code a bit.
- Uses the new Java 11 HTTP client, provides async support for downloading multiple files
- Progress handling (TODO needs hooking up to gradle)
- Better compression support. Handled by [Methanol](https://mizosoft.github.io/methanol/) 
- Unit tested with a real web server.

src/main/java/net/fabricmc/loom/LoomGradleExtension.java

@@ -46,6 +46,7 @@ import net.fabricmc.loom.configuration.providers.minecraft.mapped.IntermediaryMi
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.NamedMinecraftProvider;
 import net.fabricmc.loom.extension.LoomFiles;
 import net.fabricmc.loom.extension.MixinExtension;
+import net.fabricmc.loom.util.download.DownloadBuilder;
 
 public interface LoomGradleExtension extends LoomGradleExtensionAPI {
 	static LoomGradleExtension get(Project project) {
@@ -106,4 +107,6 @@ public interface LoomGradleExtension extends LoomGradleExtensionAPI {
 	List<AccessWidenerFile> getTransitiveAccessWideners();
 
 	void addTransitiveAccessWideners(List<AccessWidenerFile> accessWidenerFiles);
+
+	DownloadBuilder download(String url);
 }