Add missing fields to the update conversation endpoint

[cbj0hns0n]

What is in this PR?

The update conversation endpoint only supported a handful of fields. This adds everything the edit event form needs: scheduled times, platforms, features, topic, and conversation type.

Changes in the codebase

• Expanded the update endpoint's validation to accept scheduledTime, scheduledEndTime, topicId, type, platforms, properties, and features
• Properties now merge with existing values instead of overwriting, so only changed keys are affected; switching conversation type also recreates the agents and adapters for that type
• conversationType is no longer marked immutable, so it can be updated on an existing event
• Added alternateName to speakers and moderators in both the create and update schemas

Documentation and automated testing

Did you:

• document any breaking changes in your commit messages?
• document your changes as comments in the code? Use TSDoc format where appropriate.
• update the README and docs to be clear and easy to use for end users and developers?
• add and/or update automated tests?
• update team documentation of any new or changed environment variables?
• add a What's New entry in content/whatsNew.ts for new features?

Testing this PR

Related to berkmancenter/nextspace#127, test steps are listed there.