Introduce ProviderFactory.

The goal is to become Provider-agnostic one day

pgpainless-core/src/main/java/org/pgpainless/provider/BouncyCastleProviderFactory.java

@@ -0,0 +1,20 @@
+package org.pgpainless.provider;
+
+import java.security.Provider;
+
+import org.bouncycastle.jce.provider.BouncyCastleProvider;
+
+public final class BouncyCastleProviderFactory extends ProviderFactory {
+
+    private static final Provider provider = new BouncyCastleProvider();
+
+    @Override
+    public Provider _getProvider() {
+        return provider;
+    }
+
+    @Override
+    public String _getProviderName() {
+        return _getProvider().getName();
+    }
+}

pgpainless-core/src/main/java/org/pgpainless/provider/ProviderFactory.java

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+package org.pgpainless.provider;
+
+import java.security.Provider;
+
+public abstract class ProviderFactory {
+
+    private static ProviderFactory FACTORY = new BouncyCastleProviderFactory();
+
+    protected abstract Provider _getProvider();
+    protected abstract String _getProviderName();
+
+    public static void setFactory(ProviderFactory factory) {
+        ProviderFactory.FACTORY = factory;
+    }
+
+    public static Provider getProvider() {
+        return ProviderFactory.FACTORY._getProvider();
+    }
+
+    public static String getProviderName() {
+        return ProviderFactory.FACTORY._getProviderName();
+    }
+
+}

pgpainless-core/src/test/java/org/pgpainless/ProviderFactoryTest.java

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+package org.pgpainless;
+
+import static junit.framework.TestCase.assertEquals;
+
+import org.junit.Test;
+import org.pgpainless.provider.ProviderFactory;
+
+public class ProviderFactoryTest {
+
+    @Test
+    public void providerFactoryDefaultIsBouncyCastleTest() {
+        assertEquals("BC", ProviderFactory.getProviderName());
+    }
+}