Merge Rust EncryptionKeyProvider and KeyPair

[ipetr0v]

This PR:

• Merges oak_crypto::encryptor::EncryptionKeyProvider and oak_crypto::hpke::KeyPair into oak_crypto::encryption_key::EncryptionKey
• Implements serialization/deserialization
• Implements encryping the private key for Key Provisioning.
• Removes public keys from EncryptionKey struct (it's only used to be put into the evidence)

Ref #4513

[conradgrobler]

I thinkg the private key already implements it: https://github.com/rozbb/rust-hpke/blob/main/src/dhkex/x25519.rs#L17-L20

[ipetr0v]

Thanks

So we don't need to do this manually

Removing the comment