Add lastModified&contentLength to SJH URLConnections (#55)

Implement last modification time and content length in URLConnection for
SJH modular class loaders. Enables support for javacpp.

javacpp tries to extract the native DLLs it is wrapping into a cache
directory. In normal operation, it retrieves the lastModifiedDate+size
from a JarURLConnection to handle conflicts. When using javacpp in
Neoforge, this fails. javacpp cannot use its JarUrlConnection special
case and will fall back to using the generic URLConnection methods
`getContentLength` and `getLastModified`, which SJH does not implement.
That leads to crashes (since javacpp can no longer detect if the DLL it
is trying to extract is different from the one already on disk).
Implementing these two methods in SJH fixes the issue.

Fixes bytedeco/javacpp#697

Author: shartte

src/main/java/cpw/mods/cl/ModularURLHandler.java

@@ -67,9 +67,35 @@ public InputStream getInputStream() throws IOException {
                 throw e.getCause();
             }
         }
+
+        @Override
+        public int getContentLength() {
+            var length = getContentLengthLong();
+            if (length < 0 || length > Integer.MAX_VALUE) {
+                return -1;
+            }
+            return (int) length;
+        }
+
+        @Override
+        public long getContentLengthLong() {
+            return provider.getContentLength(url);
+        }
+
+        @Override
+        public long getLastModified() {
+            return provider.getLastModified(url);
+        }
+
     }
     public interface IURLProvider {
         String protocol();
         Function<URL, InputStream> inputStreamFunction();
+        default long getLastModified(URL url) {
+            return 0;
+        }
+        default long getContentLength(URL url) {
+            return -1;
+        }
     }
 }

src/main/java/cpw/mods/cl/UnionURLStreamHandler.java

@@ -33,4 +33,26 @@ public Function<URL, InputStream> inputStreamFunction() {
 
         };
     }
+
+    @Override
+    public long getLastModified(URL u) {
+        try {
+            return Files.getLastModifiedTime(Paths.get(u.toURI())).toMillis();
+        } catch (URISyntaxException e) {
+            throw new RuntimeException(e);
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+    }
+
+    @Override
+    public long getContentLength(URL u) {
+        try {
+            return Files.size(Paths.get(u.toURI()));
+        } catch (URISyntaxException e) {
+            throw new RuntimeException(e);
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+    }
 }