WIP: examples: adds an agent storing keys on TPMs

[baloo]

This relies on APIs I'd like to add into tss-esapi crate

TODO:

• plug tokio graceful shutdown, it currently leaks objects in the TPM.

[wiktor-k]

I like the idea and where this is going. I've been thinking about the same with a couple of different design decisions:

I'd use create_primary with NIST-P256 curve and a static set of parameters on start (e.g. static unique value). That'd guarantee that the TPM-backed key is always the same and the key generation via EC is fast enough not to notice anyway. That'd remove the need to store any state making the code a bit cleaner.

[baloo]

I'd use create_primary with NIST-P256 curve and a static set of parameters on start (e.g. static unique value). That'd guarantee that the TPM-backed key is always the same and the key generation via EC is fast enough not to notice anyway. That'd remove the need to store any state making the code a bit cleaner.

Yeaaah, but then I need a completely different abstraction over in tss-esapi. (I want to demo the use of a signer).
(but I'd prefer to do that too, I could then completely remove the need for the state file)