Make the key crate does not use 0x prefix in serialized form

We decided not to use 0x in the hex representation of binary data.
The current implementation of Deserialize of ed25519 and x25519 is using 0x as a prefix.

Search the key crate for the Deserialize implementations for ed25519 and x25519, then inspect how their binary data is converted to hexadecimal. Confirm the expected representation excludes the 0x prefix, update both implementations, and run the relevant crate tests or add coverage showing the serialized form without that prefix.

Vom Indexierungsmodell aus dem Issue-Text verfasst.