Add generic usage event used for iOS/macOS workflows + generic exception event

[eliasyishak]

Related to tracker issue:

• ☂️ package:unified_analytics implementation in flutter tool flutter/flutter#128251

This PR adds 2 events here, one for the generic UsageEvent in flutter, and another for the sendException method in flutter.

For the UsageEvent, you can refer to this sheet which shows all the call sites for that event. I found that all of these events were apple related so I consolidated them down into just one event. The event created for is called Event.appleUsageEvent

For the sendException method, we are only sending the exception's runtimeType as a string from flutter, so i created the Event.exception constructor which takes just one parameter for the exception string. Because this not specific to only flutter, we can have other dash tools also use this event to track exceptions in their workflows.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:unified_analytics	5.5.0	ready to publish	unified_analytics-v5.5.0
package:graphs	2.3.2-wip	WIP (no publish necessary)
package:extension_discovery	2.0.1-wip	WIP (no publish necessary)
package:cli_config	0.1.2	already published at pub.dev

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