* add: IsServiceAccount to SignedInUser and IsRealUser
* fix: linting error
* refactor: add function IsServiceAccountUser()
By adding the function IsServiceAccountUser() we use it to identify for
ServiceAccounts in the HasUniqueID() since caching is built up on having
a uniqueID, see comment: https://github.com/grafana/grafana/pull/58015#discussion_r1011361880