Implement key retrieval in encryption service

Implement key retrieval in encryption service

[Feature] Encryption/decryption service implementation
[Solution] Encryption service sends a key request, CKM service retrieves the
key and returns it to Encryption service.

[Verification] Run ckm-tests --group=CKM_ENCRYPTION_DECRYPTION and observe
journalctl -f. TED_0010_encrypt_invalid_param_list should print:
"Attempt to retrieve key failed with error: -15" (5 times)
other failing tests should print:
"Encryption/decryption not yet supported"

Change-Id: I56dc8a08ba211e996295f962da12193027c1a78c