Implement TestEventTriggerDelegate for OTA requestor

[Damian-Nordic]

Problem

Testing OTA requestor is tough because it may involve waiting for very long time before the requestor sends another QueryImage command.

Proposed Solution

Implement TestEventTriggerDelegate that could be used to trigger OTA query without waiting for the next timer expiry.

The delegate should handle the following event triggers: 0x0100_0000_0000_01<fi> where <fi> is Fabric Index of the OTA provider to query, or <00> if the OTA provider should be automatically selected.
The trigger key can be run-time configurable, although most examples can simply use a constant.