Expose enum related methods in ClassDB

[timothyqiu]

This PR exposes 4 enum related methods in ClassDB:

• class_has_enum(class, name) to tell whether name is an enum in class
• class_get_enum_list(class) to retrieve a list of enums in class
• class_get_enum_constants(class, enum) to retrieve a list of all valid keys class.enum has.
• class_get_integer_constant_enum(class, name) to get which enum the constant name belongs to.

Useful when writing plugins.

extends SceneTree

func _init():
        # HTTPClient has enum Method? True
	print("HTTPClient has enum Method? ", ClassDB.class_has_enum("HTTPClient", "Method"))

        # Enums in HTTPClient: [Status, Method, ResponseCode, ConnectFlags]
	print("Enums in HTTPClient: ", ClassDB.class_get_enum_list("HTTPClient"))

        # Values in HTTPClient.Method: [METHOD_GET, METHOD_HEAD, METHOD_POST, METHOD_PUT, METHOD_DELETE, METHOD_OPTIONS, METHOD_TRACE, METHOD_CONNECT, METHOD_PATCH, METHOD_MAX]
	print("Values in HTTPClient.Method: ", ClassDB.class_get_enum_constants("HTTPClient", "Method"))

        # METHOD_GET belongs to enum: Method
	print("METHOD_GET belongs to enum: ", ClassDB.class_get_integer_constant_enum("HTTPClient", "METHOD_GET"))

	quit()

[mhilbrunner]

I would have needed something like this in multiple cases I had to abandon in the past.

Implementation looks good and unsurprising.