is hostname in UserIdentity is fixed to '%'

[youngsofun]

Summary

https://docs.databend.com/sql/sql-commands/ddl/user/user-create-user

if it is, we can

1. refactor code, e.g. remove the hostname field in UserIdentity.
2. safely use user_name itself to play the role of UserIdentity in some cases？

[youngsofun]

cc @TCeason @sundy-li

[TCeason]

cc @TCeason @sundy-li

#15652 (comment)

[youngsofun]

but seem current sql parse can not accept <user_name>@'<hostname>' at all.

but it seems these users can not be used at all

1. current sql parse can not accept <user_name>@'<hostname>'
2. current auth only care about <user_name> too.

maybe we can simple keep some key parser code to not report error when list these old users?