fix: jsonschema, variables maybe in string type, e.g. '${get_variables()}'

httprunner/__init__.py

@@ -1,4 +1,4 @@
-__version__ = "2.5.2"
+__version__ = "2.5.3"
 __description__ = "One-stop solution for HTTP(S) testing."
 
 __all__ = ["__version__", "__description__"]

httprunner/loader/schemas/common.schema.json

@@ -23,6 +23,12 @@
                         "maxProperties": 1,
                         "minProperties": 1
                     }
+                },
+                {
+                    "type": "string",
+                    "examples": [
+                        "${get_variables()}"
+                    ]
                 }
             ]
         },