Commit b7896b3153 moved events.go from the 'authenticate' package to
'internal/authenticateflow' in order to avoid an import cycle. However
this location is not actually suitable, as the hosted authenticate
service refers to AuthEvent and AuthEventFn.
Move events.go back out from under 'internal', to a new package
'authenticate/events'. This should still avoid an import cycle between
'authenticate' and 'internal/authenticateflow', while also allowing the
hosted authenticate service to use the events types.
Consolidate all logic specific to the stateless authenticate flow into a
a new Stateless type in a new package internal/authenticateflow. This is
in preparation for adding a new Stateful type implementing the older
stateful authenticate flow (from Pomerium v0.20 and previous).
This change is intended as a pure refactoring of existing logic, with no
changes in functionality.
