Make aws/aws-sdk-php dependency optional

Problem/Motivation

While investigating the size of our vendor folder, we noticed that vendor/aws/ was one of the largest subfolders (~50 MB).
I see this dependency is only used by the ckeditor5_premium_features_ai_assistant submodule and we don't use it.

Steps to reproduce

Proposed resolution

Instead of requiring it, it could be suggested: https://getcomposer.org/doc/04-schema.md#suggest
The code using it would probably need to be adapted to throw a clean error when the library is not there.

Remaining tasks

User interface changes

API changes

Data model changes