[ML] Transforms: Consolidate edit/create form code

[walterra]

The forms to create a transform in the creation wizard and the one in the flyout on the transform list to edit a transform at the moment use different approaches to state management and form validation. This issue is used to track progress on consolidation of that code as well as some other minor improvements to form validation.

• edit/create use same validators
• edit/create use same state management
• add a feature to form validation that allows checking if a value had been previously set and is not removable [ML] Transforms: Extend editing and creation options. #77370 (comment)
• support dependencies on required fields for updates (e.g. the dest.index field is required when updating dest.pipeline)
• fix an issue where a form value could end up as NaN in the transform creation wizard (should be solved by consolidating with the edit transform code) [ML] Transforms: Extend editing and creation options. #77370 (comment)
• improve functional test utility clickTransformEditAccordionAdvancedSettings() [ML] Transforms: Extend editing and creation options. #77370 (comment)
• In the creation wizard, the form's state gets populated with default values. Previously, these values ended up being sent along with the creation request. We fixed passing along those default values in [ML] Transforms: Fix to not pass on default values in configurations. #129091, but ideally the form shouldn't be populated at all, instead input field should stay empty and the in place helper text should mention the default value, like we do in the flyout to edit a transform.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)