Added "id" parameter for make_forecaseting_frame so that it will not …

[heib6xinyu]

…have hard coded 'id' in the result.

This is my edition of make_forecasting_frame of the issue #1077 .
I added id parameter to the function, so the result will be (a, t1) instead of (id, t1) for item that has the id of a.

[nils-braun]

Hi @heib6xinyu - thanks for providing a PR!
As we discussed on the linked issue, the function make_forecasting_frame is only meant to be used for singular time series (as the name "frame" suggests). While having an id parameter is technically possible (as you have shown in your PR), it might make people think the function should be used for timeseries with multiple ids (because all other functions which allow for different id columns have a parameter to control the id column).
Therefore I would not like to add the id parameter. Also, it would be a breaking API change of our public API, so we need to be very careful about it.

Would it be possible for you to do the change from id to e.g. a as a post-processing step in your code?