change_type: breaking
component: session
note: Move `session.id` and `session.previous_id` from body fields to event attributes, and yamlize `session.start` and `session.end` events.
issues: [1845]
subtext: >
  As part of the ongoing migration of event fields from LogRecord body to extended/complex attributes, the
  `session.start` and `session.end` events have been redefined.
