text: Title: [ENH] Support detection for number of clusters
  Body: In a non-time-series situation, identification of optimal number of clusters is quite common, and people usually do elbow curves with SSW, maximum silhouette score, etc. It will be good to have such capability in `sktime` to find the optimal number of groups while performing time series clustering.
  
  1. The user must be able to specify a metric of choice, e.g. intertia (available as model attributes), silhouette score (https://tslearn.readthedocs.io/en/stable/gen_modules/clustering/tslearn.clustering.silhouette_score.html), etc.
  2. User may choose to pass an array of number of clusters to try.
  3. Alternatively, user may specify minimum and maximum number of clusters.