[MGPG-135] Support Overriding / Enhance the signer in AbstractGpgMojo (#112)

Currently AbstractGpgMojo has a fixed set of supported signers (and how
they are constructed), it would be good to allow extensions (e.g. Tycho
is also using AbstractGpgMojo) to possibly override that aspect.

This extracts the creation of the signer into a (protected) method so
different types or alternative implementations can be used.

Co-authored-by: Christoph Läubrich <christoph@laeubi-soft.de>

Author: laeubi

src/main/java/org/apache/maven/plugins/gpg/AbstractGpgMojo.java

@@ -337,20 +337,7 @@ private void logBestPracticeWarning(String source) {
     }
 
     protected AbstractGpgSigner newSigner(MavenProject mavenProject) throws MojoFailureException {
-        AbstractGpgSigner signer;
-        if (GpgSigner.NAME.equals(this.signer)) {
-            signer = new GpgSigner(executable);
-        } else if (BcSigner.NAME.equals(this.signer)) {
-            signer = new BcSigner(
-                    session.getRepositorySession(),
-                    keyEnvName,
-                    keyFingerprintEnvName,
-                    agentSocketLocations,
-                    keyFilePath,
-                    keyFingerprint);
-        } else {
-            throw new MojoFailureException("Unknown signer: " + this.signer);
-        }
+        AbstractGpgSigner signer = createSigner(this.signer);
 
         signer.setLog(getLog());
         signer.setInteractive(settings.isInteractiveMode());
@@ -395,6 +382,24 @@ protected AbstractGpgSigner newSigner(MavenProject mavenProject) throws MojoFail
         return signer;
     }
 
+    protected AbstractGpgSigner createSigner(String name) throws MojoFailureException {
+        AbstractGpgSigner signer;
+        if (GpgSigner.NAME.equals(name)) {
+            signer = new GpgSigner(executable);
+        } else if (BcSigner.NAME.equals(name)) {
+            signer = new BcSigner(
+                    session.getRepositorySession(),
+                    keyEnvName,
+                    keyFingerprintEnvName,
+                    agentSocketLocations,
+                    keyFilePath,
+                    keyFingerprint);
+        } else {
+            throw new MojoFailureException("Unknown signer: " + name);
+        }
+        return signer;
+    }
+
     private boolean isNotBlank(String string) {
         return string != null && !string.trim().isEmpty();
     }