[ENH] Change fitted estimators and ensemble collections from protected to public

[MatthewMiddlehurst]

Describe the feature or idea you want to propose

Currently in a lot of estimators we fit an sklearn estimator i.e. _estimator or form an ensemble of estimators i.e. _ensemble or _classifiers but these are protected (starting with an _).

Theres not much reason for them to be protected IMO.

Describe your proposed solution

Change fitted estimator attributes from protected to public using our fitted attribute convention i.e. from _estimator to estimator_.

These should then be properly documented in the estimator docstring under "Attributes"

Describe alternatives you've considered, if relevant

No response

Additional context

No response

[TonyBagnall]

see #2350 for an example

[vedpawar2254]

@@aeon-actions-bot assign @vedpawar2254