Add session.change event

[breedx-splk]

Changes

This adds semantic conventions for an event called session.change.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [x] schema-next.yaml updated with changes to existing conventions. (I think this is N/A due to automation lacking for events)