Interruptible APIs for key agreement use cases

[athoelke]

Add interruptible operations for psa_generate_key(), psa_export_public_key() and psa_key_agreement().

This provides the new API elements, and updates the Functionality chapter to cover this simpler interruptible pattern, as well as the complex interruptible multi-part operations used for signature and verification from #107.

This is based on top of #107. The first unique commit is https://github.com/ARM-software/psa-api/commit/57ae95c9dd6cd5aed55806d1aca3c021fc5dd8e7.

A preview of the PDF rendering of this PR: IHI0086-PSA_Certified_Crypto_API-1.3.0-interruptible-operations-draft3.pdf

Fixes #198