Add interceptor hooks to TrackService and ExceptionHandler

[ghsolomon]

Context: we are using Datadog RUM in client applications and looking to include

• Tracked user activity as "Custom Actions"
• Handled exceptions as "Custom Errors"

Additionally, it would be nice to append session information, such as the sessionReplayLink, to our user activity and exception data.

Consider adding the ability to intercept calls to ExceptionHandler.handleException and TrackService.track so that applications can be notified of errors / activities and possibly enhance them with additional metadata.

[amcclain]

To be looked at in context of #3802