[core] add missing platform function for OT_PLATFORM_KEY_REF (#9620)

This commit contains the following changes:
- Adds `CMake` option `OT_PLATFORM_KEY_REF`, corresponding to the
  existing `OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE`
  config.
- Add empty implementations of `otPlatCryptoEcdsa{}` under the
  simulation platform.
- Modifies `test_platform.cpp` to ensure that the key-ref related APIs
  are defined only when the key reference feature is enabled.
- Updates `toranj/build.sh` script to provide the option to enable the
  key reference feature.
- Updates GitHub workflow to validate build with key reference feature
  under simulation platform, ensuring that future PRs are validated
  with this feature enabled.

tests/unit/test_platform.cpp

@@ -478,8 +478,6 @@ bool otPlatCryptoHasKey(otCryptoKeyRef aKeyRef)
     return false;
 }
 
-#endif // OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE
-
 otError otPlatCryptoEcdsaGenerateAndImportKey(otCryptoKeyRef aKeyRef)
 {
     OT_UNUSED_VARIABLE(aKeyRef);
@@ -517,6 +515,8 @@ otError otPlatCryptoEcdsaVerifyUsingKeyRef(otCryptoKeyRef                    aKe
     return OT_ERROR_NONE;
 }
 
+#endif // OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE
+
 otError otPlatRadioSetCcaEnergyDetectThreshold(otInstance *aInstance, int8_t aThreshold)
 {
     OT_UNUSED_VARIABLE(aInstance);