record saml_idp_sessions event

[flyinghermit]

The SAML IdP auth handler emits audit event saml.idp.auth.
The event is based on event type SAMLIdPAuthAttempt.
This PR handles successful auth attempt audit event as saml_idp_session start event SessionStartEvent and adds the event counter to the user activity record.

Sample user activity record with the saml_idp_sessions:

 {
  "report_uuid": "McxKGo9pSumU8DPPXeRDzg==",
  "cluster_name": "Qe8aKevkim6h3hReJvEnWzdVoCW49OjE2qYVEqlU4Ng=",
  "reporter_hostid": "Gl+XkAEDry4dQszdh/U8dAytD/WOadbWioX1ecu3ZIE=",
  "start_time": {
   "seconds": 1739577720
  },
  "records": [
   {
    "user_name": "i/P37sO9T0Xcd6HFV9N0AM05h1faMuyP8FQh7wASx0I=",
    "user_kind": 1,
    "saml_idp_sessions": 2
   }
  ]
 }

Cloud prehog twin PR https://github.com/gravitational/cloud/pull/12092
Part of https://github.com/gravitational/teleport.e/issues/5946

[smallinsky]

LGTM when the https://github.com/gravitational/teleport/pull/52202/files#r1959507472 will be address.