Remove the hack where we pass keys through PSA Crypto to get the public key out

[dreemkiller]

Requested feature In several places in the code, we load the asymmetric key pair into PSA Crypto and then immediately extract the public key from PSA Crypto. This is done because none of the other crypto libraries support extracting the public component from the keypair. We should find a better way to do this.

Motivation This is super hacky, and it embarrasses me that I wrote it, but... someone else should fix it.

Additional context You can find one example of this in nitro-root-enclave/src/main.rs, near let_device_public_key = There are other places this is done as well (jalisco and sonora also do this, I think)

[egrimley-arm]

Is this issue still valid? Could the pointer to the example be updated?

(According to #26, jalisco is now sgx-root-enclave, and sonora is now trustzone-root-enclave, but I don't know where nitro-root-enclave is.)