[TF-M] Supporting encryption with ITS

Hello all,

I wanted to let you know that I made the PR which adds encryption on ITS files using the methodology that we discussed before. The encryption is happening in a transparent way for the user, and I tried to avoid major changes in the ITS filesystem. Please add yourself as a reviewer and provide feedback if you think that this is an interesting use case for you.

https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/15096

Regards, Georgios Vasilakis