feat: add start and endtime for sessions

[ZohebShaikh]

And the data is added here:-

    "sessions": {
      "11": {
        "beamline": "b1",
        "proposal_number": 2124,
        "visit_number": 1,
        "start_time": 1778761180,
        "end_time": 1779761180
      },

Policy example with this:-

# We will get the unix timestamp from the database
some_session_end_time := 1779759400 

seconds_to_ns := 1000000000

default allow := false

allow if {
	time.now_ns() <= some_session_end_time * seconds_to_ns
}

[Alexj9837]

Looks good to me.