feat: service get account private key for JWT encode (google-walle#112)

src/Credentials/ServiceAccountCredentials.php

@@ -333,6 +333,19 @@ public function getClientName(callable $httpHandler = null)
         return $this->auth->getIssuer();
     }
 
+    /**
+     * Get the private key from the keyfile.
+     *
+     * In this case, it returns the keyfile's private_key key, needed for JWT signing.
+     *
+     * @param callable $httpHandler Not used by this credentials type.
+     * @return string
+     */
+    public function getPrivateKey(callable $httpHandler = null)
+    {
+        return $this->auth->getSigningKey();
+    }
+
     /**
      * Get the quota project used for this API request
      *

tests/Credentials/ServiceAccountCredentialsTest.php

@@ -369,6 +369,13 @@ public function testReturnsClientEmail()
         $this->assertEquals($testJson['client_email'], $sa->getClientName());
     }
 
+    public function testReturnsPrivateKey()
+    {
+        $testJson = $this->createTestJson();
+        $sa = new ServiceAccountCredentials('scope/1', $testJson);
+        $this->assertEquals($testJson['private_key'], $sa->getPrivateKey());
+    }
+
     public function testGetProjectId()
     {
         $testJson = $this->createTestJson();