replace enum datastructure to a simple class structure in yrpc

[yingshaoxo]

Old:

class User_Type(Enum):
    user = "user"
    admin = "admin"

New:

class User_Type():
    user: str = "user"
    admin: str = "admin"

[yingshaoxo]

Instead making a big change, I added a comment to all those enum class:

# yingshaoxo: I strongly recommend you use enum as a string type in other message data_model