[nrf fromlist][crypto] Extend the OperationalKeystore API to allow migration

- Extended the OperationalKeystore API by mechanism for migration of
operational keys stored in one Operational Keystore implementation
to another.

- Extended the OperationalKeystore API by exporting keypair for Fabric.

- Added Unit tests to PersistentStorageOpKeyStore and PSAOpKeystore regarding
the new OperationalKeystore for migration and exporting OpKeys.

Added first unit tests, created export method

Aligned NXP and Silabs platform to the recent OperationalKeystore changes.

Author: ArekBalysNordic

src/crypto/OperationalKeystore.h

@@ -152,6 +152,7 @@ class OperationalKeystore
      * @retval CHIP_ERROR_NOT_IMPLEMENTED if the exporting is not implemented for the cryptography backend.
      * @retval CHIP_ERROR_INVALID_FABRIC_INDEX if there is no keypair found for `fabricIndex`.
      * @retval CHIP_ERROR_PERSISTED_STORAGE_VALUE_NOT_FOUND if there is no keypair found for `fabricIndex`.
+     * @retval CHIP_ERROR_BUFFER_TOO_SMALL if `keyPair` buffer is too small to store the read out keypair.
      * @retval other CHIP_ERROR value on internal storage or crypto engine errors.
      */
     virtual CHIP_ERROR MigrateOpKeypairForFabric(FabricIndex fabricIndex, OperationalKeystore & operationalKeystore) const