[XY axis] Improve expression with explicit params

[stratoula]

Describe the feature:
Right now the vis_type_xy is using a JSON visConfig which is not ideal for other teams to reuse it.
We should remove the JSON blob and use explicit params instead.

A good example of this is vis_data_table or vis_type_metric that they have already solved this problem and use explicit params instead of the visConfig json blob.

Acceptance criteria/testing scenarios:

• This should only affect the xy axis plugin and not the vislib one.
• Test backwards compatibility
• Test that nothing has been broken as XY axis has a quite big list of params.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[stratoula]

Same applies for vis_type_pie. I will take care of it here #83929

cc @ppisljar