ENH: Request for `Index.to_string` & `MultiIndex.to_string`
#### Is your feature request related to a problem?

I'd like to request `to_string` API support in `Index` & `MultiIndex`, functionality being similar to the existing `DataFrame.to_string` & `Series.to_string`.

#### Describe the solution you'd like

`to_string` provides flexibility to modify/control the string representation of pandas objects, we can do this currently with `DataFrame` & `Series` but the API support is not yet there for `Index` & `MultiIndex`.

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_string.html
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.to_string.html