[DI] Use interruptible ECDH in TLS

[mpg]

Once PSAhas gained interruptible functions for ECDHE, these functions should be used by TLS when MBEDTLS_USE_PSA_CRYPTO is enabled, in all places that currently support restartable operations using the legacy API (that's currently just 1.2 with the ECDHE-ECDSA key excahnge), in order to avoid feature gaps in PSA-based builds.

This task is to investigate how to achieve that goal, and break the work down in reasonnably-size and testable steps.

Execution of the work will depend on #7293 and the tasks created from it being completed.