Make segments schema not strict (deprecate schema) and change the structure of it

Problem/Motivation

New arguments feature ✨ Allow to pass variables to aiprompt programatically Active have very simple schema:

    arguments:
      type: sequence
      label: 'Arguments'
      sequence:
        type: mapping
        label: 'Argument'
        mapping:
          '*':
            type: ignore
            label: 'Arbitrary setting'

Would be nice to do the same for segments.
There is little benefit of having strict schema for prompt 'configuration entities' when the same for 'content entity' does not have a schema at all.
Benefits of not having a schema:

• Simpler data structure and simpler code, like in arguments feature example.
• Often there would be no need to write update hooks when changing data structure.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes