Explicitly handle overflow when updating openhcl/underhill_attestation_protocol's vmgs::KeyProtector active_kp field

[mattbodd]

KeyProtectors as defined in openhcl/openhcl_attesation_protocol (link) maintain the active KP index as a u32.

In openhcl/underhill_attestation/src/lib.rs, within persist_all_key_protectors, active KP is updated with a straightforward addition assignment operation, which will implicitly wrap when adding to u32::MAX. All current usages of the active KP index us modulo arithmetic which will handle arithmetic overflow gracefully and produce desired behavior.

Two suggestions for improvement

1. Make active_kp an enum with ingress and egress variants that desugars to u32 (for VMGS file compatibility)
   • Pros: clearer than using an arbitrary index value, not susceptible to overflow
   • Cons: requires a bit of under-the-hood complexity to allow FromBytes and AsBytes to work for active_kp to be represented as u32 when reading/writing to an actual VMGS
2. Explicitly use u32::wrapping_add (link) to make the intent clearer
   • Pros: makes wrapping behavior expectation explicit, minimal change
   • Cons: maintains implicit requirement for usage of active_kp to handle wrapping behavior

[mattbodd]

I can't assign this to myself, but I'll take ownership of this issue