text: Title: `TimeSeries.plot` Title as parameter
  Body: **Is your feature request related to a current problem? Please describe.**
  The `TimeSeries.plot` method sets the plot title to the name of the underlying xarray's name.
  
  However, it is unclear to me (and to my knowledge not documented) how to set this name without touching the private `._xa`.
  
  In my experience, the name attribute is not set, if it is not constructed directly from a named xarray, which is rarely the case.
  
  **Describe proposed solution**
  
  I propose a new parameter for the plot function, that can overwrite this behaviour.
  
  - title (str): Set a title for the figure. Default is None, which plots the name of the underlying xarray (if any)
  
  https://github.com/unit8co/darts/blob/71a1902b1c28b579328984cd3a37349cd9a9aba9/darts/timeseries.py#L4257
  would change then to:
  `ax.set_title(title if title else self._xa.name)`
  
  **Describe potential alternatives**
  
  - Extend the TimeSeries API so a name can be set. E.g. `TimeSeries.set_name(self, name: str)`
  - Provide a name parameter for all TimeSeries construction methodes, such as e.g. `TimeSeries.from_array(..., name: str)`
  
  I would prefer proposed solution which is more flexible to the user
  
  **PR**
  
  I can to a PR for that
  
  