RFC: AES GCM mode support

[ozersa]

Currently MCUboot and imgtool does not support AES-GCM mode, https://github.com/mcu-tools/mcuboot/blob/main/scripts/imgtool/image.py#L61

We need AES-GCM support both on imgtool and mcuboot.
This is RFC ticket for this purpose.

Useful link: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation