RTC Slots: Refactoring of membership event parsing and handling

[Half-Shot]

Split out from #5056

Part of a series of PRs intending to support Matrix RTC MSC4143 slot behaviours.

This one focuses on refactoring the way we handle membership from different eras of Matrix RTC, as currently we need to support both the older and newer forms of MSC4143.

This does NOT implement slots itself, but forms the basis for doing so.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).