Dem test event triggers

[r-recchia]

This is the second PR of issue 389. It addresses the second bullet listed in the issue about DEM (Device Energy Management)

• update matter_testing send_test_event_triggers to encode the endpoint from the --endpoint argument within the test event trigger
• update SDK implementation of the TestEventTriggerHandlers to account for endpoints
• implement an override flag (--use-legacy-test-event-triggers) in the test to allow sending test event triggers with EP0 for older devices - this shall be equivalent to "first endpoint" on the device, as it is handled now and will allow newer tests to be used with older devices
• add within-test documentation about test event triggers to enable debugging of both unimplemented and legacy test event triggers at the test event

This PR updates TestEventTriggerDelegate.h and DeviceEnergyManagementTestEventTriggerHandler.h files. They take the endpoint target into account.

Testing

WIP