feat: Support multiple round-robin host selection

This PR is part of a stack, managed by mrge:

What does this PR do?

Fixes #11953
/claim #11953
Adds support for selecting multiple hosts in round-robin event types.
Modifies getLuckyUser to return either a single host (if multipleRoundRobinHosts is not specified or is 1) or multiple hosts (if multipleRoundRobinHosts > 1).
Updates the event type metadata schema (EventType.metadata) to include multipleRoundRobinHosts.
Adds comprehensive test coverage for multiple host selection scenarios.
Enhances the booking flow so that multiple hosts can be assigned to a single booking if configured.

I have self-reviewed the code (A decent size PR without self-review might be rejected).
I have updated the developer docs in /docs if this PR makes changes that would require a [documentation change](https://cal.com/docs). (If N/A, check this box and write “N/A” here.) N/A
I confirm automated tests are in place that prove my fix is effective or that my feature works.

Setup: Pull this branch and run any necessary migrations.
Create or edit an Event Type:
- Under the “Round Robin” settings, ensure there are no fixed hosts.
- Add multiple users as round-robin hosts.
- In the EventType.metadata, set multipleRoundRobinHosts to the desired number (e.g., 2 or 3).
Book an Event:
- Book the event as a normal user and confirm that multiple hosts are indeed selected.
- Verify in the database or in the logs that the expected hosts are assigned to the booking.
Check Calendar/Email/Webhook (if enabled):
- Each of the assigned hosts should receive notification (and appear as attendees if your configuration supports that).
- Any configured webhooks should fire as usual, including references to all selected hosts.

No additional environment variables are required for this feature.