[Breaking Change] Add support for Document Snapshots in Firestore Que…

…ry Cursors (#1162)

cc @schmidt-sebastian

Extracted from #923 and updated to address pull request comments.

Breaking change is the standardization in Query of using `InvalidArgumentException`, replacing various throws of `BadMethodCallException`.

Closes #851.

Firestore/src/DocumentSnapshot.php

@@ -294,7 +294,10 @@ public function get($fieldPath)
                 break;
             } else {
                 if (!isset($fields[$part])) {
-                    throw new \InvalidArgumentException('field path does not exist.');
+                    throw new \InvalidArgumentException(sprintf(
+                        'Field path `%s` does not exist.',
+                        $fieldPath
+                    ));
                 }
 
                 $fields = $fields[$part];

Firestore/src/FieldValue.php

@@ -84,4 +84,19 @@ public static function serverTimestamp()
     {
         return '___google-cloud-php__serverTimestamp___';
     }
+
+    /**
+     * Check if the given value is a sentinel.
+     *
+     * @param string $value
+     * @return bool
+     * @access private
+     */
+    public static function isSentinelValue($value)
+    {
+        return in_array($value, [
+            self::deleteField(),
+            self::serverTimestamp()
+        ]);
+    }
 }