[cbor] Use libcoset/libciborium to convert ed25519 pub key to cbor

This replaces the manual serialization and makes the code easier
to read and maintain.

The test diced_open_dice_cbor_test ensures that the result is
exactly the same as before.

Test: atest diced_open_dice_cbor_test
Change-Id: Ice6ed3f71875c52a11e1fe4c36d5ff0d8ab9ae6d
2 files changed