Make the naming of the remote attestation public key fields consistent

[conradgrobler]

At the moment a public key is generated during startup of Oak Functions. This key represents the identity of the instance and will be inlcuded in the remote attestation quote.

When using interactive remote attestation this key will be used for signing the handshake transcripts. For non-interactive attestation it will be used for hybrid encryption.

We should find naming for the various instances that are consistent, while preferably still clarifying the intended us.

See https://github.com/project-oak/oak/pull/3171#discussion_r961103996 for more context