[GPG] Provide a factory for the BouncyCastleGpgSigner

Otherwise client code has no way to ever create an instance without
using internal non-API.

Change-Id: I6201f98d4b1704a053159967b8adacd98e368522
Signed-off-by: Thomas Wolf <[email protected]>

Author: Thomas Wolf

org.eclipse.jgit.gpg.bc/META-INF/MANIFEST.MF

@@ -34,5 +34,6 @@ Import-Package: org.bouncycastle.asn1;version="[1.65.0,2.0.0)",
  org.eclipse.jgit.transport;version="[5.11.0,5.12.0)",
  org.eclipse.jgit.util;version="[5.11.0,5.12.0)",
  org.slf4j;version="[1.7.0,2.0.0)"
-Export-Package: org.eclipse.jgit.gpg.bc.internal;version="5.11.0";x-friends:="org.eclipse.jgit.gpg.bc.test",
+Export-Package: org.eclipse.jgit.gpg.bc;version="5.11.0",
+ org.eclipse.jgit.gpg.bc.internal;version="5.11.0";x-friends:="org.eclipse.jgit.gpg.bc.test",
  org.eclipse.jgit.gpg.bc.internal.keys;version="5.11.0";x-friends:="org.eclipse.jgit.gpg.bc.test"

org.eclipse.jgit.gpg.bc/src/org/eclipse/jgit/gpg/bc/BouncyCastleGpgSignerFactory.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2021 Thomas Wolf <[email protected]> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.gpg.bc;
+
+import org.eclipse.jgit.gpg.bc.internal.BouncyCastleGpgSigner;
+import org.eclipse.jgit.lib.GpgSigner;
+
+/**
+ * Factory for creating a {@link GpgSigner} based on Bouncy Castle.
+ *
+ * @since 5.11
+ */
+public final class BouncyCastleGpgSignerFactory {
+
+	private BouncyCastleGpgSignerFactory() {
+		// No instantiation
+	}
+
+	/**
+	 * Creates a new {@link GpgSigner}.
+	 *
+	 * @return the {@link GpgSigner}
+	 */
+	public static GpgSigner create() {
+		return new BouncyCastleGpgSigner();
+	}
+}