Support control character in config (sonic-net#217)

Why I did it
sonic-net/sonic-buildimage#16957
Above PR introduced "\n" in config_db, and current GNMI implementation does not support such config.

How I did it
Update yang validation part to read config from file.

How to verify it
Run GNMI end to end test.

sonic_data_client/mixed_db_client.go

@@ -1133,14 +1133,16 @@ import json
 
 yang_parser = sonic_yang.SonicYang("/usr/local/yang-models")
 yang_parser.loadYangModel()
-text = '''%s'''
-
-try:
-    yang_parser.loadData(configdbJson=json.loads(text))
-    yang_parser.validate_data_tree()
-except sonic_yang.SonicYangException as e:
-    print("Yang validation error: {}".format(str(e)))
-    raise
+filename = "%s"
+with open(filename, 'r') as fp:
+	text = fp.read()
+
+	try:
+		yang_parser.loadData(configdbJson=json.loads(text))
+		yang_parser.validate_data_tree()
+	except sonic_yang.SonicYangException as e:
+		print("Yang validation error: {}".format(str(e)))
+		raise
 `
 
 func (c *MixedDbClient) SetIncrementalConfig(delete []*gnmipb.Path, replace []*gnmipb.Update, update []*gnmipb.Update) error {