feat: service get account private key for JWT encode

src/Credentials/ServiceAccountCredentials.php

@@ -343,6 +343,18 @@ 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.
+     *
+     * @return string
+     */
+    public function getPrivateKey()
+    {
+        return $this->auth->getSigningKey();
+    }
+
     /**
      * Get the quota project used for this API request
      *

src/Credentials/ServiceAccountJwtAccessCredentials.php

@@ -217,6 +217,18 @@ 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.
+     *
+     * @return string
+     */
+    public function getPrivateKey()
+    {
+        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();

tests/Credentials/ServiceAccountJwtAccessCredentialsTest.php

@@ -492,6 +492,14 @@ public function testReturnsClientEmail()
         $sa = new ServiceAccountJwtAccessCredentials($testJson);
         $this->assertEquals($testJson['client_email'], $sa->getClientName());
     }
+
+    public function testReturnsPrivateKey()
+    {
+        $testJson = $this->createTestJson();
+        $sa = new ServiceAccountJwtAccessCredentials($testJson);
+        $this->assertEquals($testJson['private_key'], $sa->getPrivateKey());
+    }
+
     public function testGetProjectId()
     {
         $testJson = $this->createTestJson();