Stage 2 of implementing interruptible ECC export public key.

This should involve implementing psa_export_public_key_iop_setup() and psa_export_public_key_iop_abort() and include any testing that we can do of the above two functions to ensure they are functionally correct.

Remember that abort should be called internally in case of any error.

Function prototypes for reference:

psa_status_t psa_export_public_key_iop_abort(psa_export_public_key_iop_t * operation);
psa_status_t psa_export_public_key_iop_setup(psa_export_public_key_iop_t * operation,
                                             psa_key_id_t key);