[Improvement] [User Impersonation] Provide support to impersonate user with name rather than User Id

[Thumimku]

Current Impersonation feature requires request subject param to initiate the impersonation. The value of the param is id of the user. Rather than using user id (which should be internal), we can provide support for user name or any unique identifier.

example: support user can enter username and start from it, rather than have a scim call to get id