Save the key context outside of the TPM

[Foxboron]

Since TPM keys are deterministic we can actually save the key context outside of the TPM isntead of a persistent handle. This saves resources on the limited hardware with the downside that the host is storing a blob.

Would need to use tpm2.ContextSave instead of tpm2.EvictControl without key handle. Then we need to use tpm2.ContextLoad before using the key again.