[telemetry] add API for TREL telemetry (#9710)

Create OT API to support trel telemetry which is supported through
platform API and also add cli support to get/reset trel counters.

Metrics we are adding are:
- trel_frames_tx
- trel_bytes_tx
- trel_frames_rx
- trel_bytes_rx
- trel_frames_tx_failed
- num_trel_peers

Metrics already supported through API:
- trel_enabled

tests/unit/test_platform.cpp

@@ -424,6 +424,10 @@ OT_TOOL_WEAK void otPlatTrelDisable(otInstance *) {}
 OT_TOOL_WEAK void otPlatTrelSend(otInstance *, const uint8_t *, uint16_t, const otSockAddr *) {}
 
 OT_TOOL_WEAK void otPlatTrelRegisterService(otInstance *, uint16_t, const uint8_t *, uint8_t) {}
+
+OT_TOOL_WEAK const otPlatTrelCounters *otPlatTrelGetCounters(otInstance *) { return nullptr; }
+
+OT_TOOL_WEAK void otPlatTrelResetCounters(otInstance *) {}
 #endif
 
 #if OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE