PSA: Support volatile secure element keys

[gilles-peskine-arm]

Implement support for volatile keys in a secure element. #3302 defines an encoding of lifetimes that makes certain keys volatile, but the code does not treat them as volatile.

This is part of a larger redesign in #3287 and should not necessarily be implemented on its own with the current structure of the code.

Goals:

• Support volatile keys in a secure element, in addition to persistent keys.
• At least smoke tests. Detailed testing is out of scope of this task.

[gilles-peskine-arm]

@stevew817 This could be a good first PR digging into the PSA code.