[ENH] Add n_timepoints in the metadata stored by BaseSeriesEstimator

[baraline]

Describe the feature or idea you want to propose

Unless mistaken, we currently don't store n_timepoints information in the metadata of BaseSeriesEstimator.

Describe your proposed solution

Axis dependent inside the function https://github.com/aeon-toolkit/aeon/blob/f5380f039babc869c0833142afb5f7e649141d52/aeon/base/_base_series.py#L102

timepoint_idx = 1 if self.axis == 1 else 0
self.metadata["n_timepoints"] = X.shape[timepoint_idx]

Describe alternatives you've considered, if relevant

No response

Additional context

No response