Add DownloaderFactory to return a specific downloader based on 2 variables.

If the system property 'downloader' is set that use that specific downloader. This is used from gradle by appending `-Ddownloader=ABCD to the command.
ABCD is one of DownloaderType.
The other variable is the static property `DEFAULT_DOWNLOADER` in DownloaderFactory, which can be easily changed as needed inside the IDE according to development needs`.

Normal workflow would be to first use the recording downloader and afterwards only use mocks, if the requests are always staying the same.

extractor/src/test/java/org/schabi/newpipe/downloader/DownloaderFactory.java

@@ -0,0 +1,30 @@
+package org.schabi.newpipe.downloader;
+
+import org.schabi.newpipe.extractor.downloader.Downloader;
+
+import java.io.IOException;
+
+public class DownloaderFactory {
+
+    private final static DownloaderType DEFAULT_DOWNLOADER = DownloaderType.REAL;
+
+    public Downloader getDownloader(String path) throws IOException {
+        DownloaderType type;
+        try {
+            type = DownloaderType.valueOf(System.getProperty("downloader"));
+        } catch (Exception e) {
+            type = DEFAULT_DOWNLOADER;
+        }
+
+        switch (type) {
+            case REAL:
+                return DownloaderTestImpl.getInstance();
+            case MOCK:
+                return new MockDownloader(path);
+            case RECORDING:
+                return new RecordingDownloader(path);
+            default:
+                throw new UnsupportedOperationException("Unknown downloader type: " + type.toString());
+        }
+    }
+}

extractor/src/test/java/org/schabi/newpipe/downloader/DownloaderType.java

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+package org.schabi.newpipe.downloader;
+
+public enum DownloaderType {
+    REAL, MOCK, RECORDING
+}