THistSaveAs txt or csv

[ferdymercury]

Explain what you would like to see improved

I open this to keep track of a suggestion by @lmoneta to save histograms as csv or txt files.
#9491 (comment)

Right now, there are all sorts of workarounds to achieve this (see below), but they are easy to forget and periodically there are new questions on the forum wrt the same thing.
A function doing
TH1::SaveAs(".txt")
would be more user-friendly.

Optional: share how it could be improved

Move TGraph implementation in #9491 to a class THistSaveAs

Setup

   ------------------------------------------------------------------
  | Welcome to ROOT 6.24/06                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Sep 02 2021, 14:20:23                 |
  | From tags/v6-24-06@v6-24-06                                      |
  | With                                                             |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

Additional context

https://root-forum.cern.ch/t/save-data-points-from-a-histogram-in-a-txt-file/49768/3
https://root-forum.cern.ch/t/save-histogram-in-txt-file/43616/13
https://stackoverflow.com/questions/28970124/cern-root-exporting-data-to-plain-text
https://root-forum.cern.ch/t/save-the-contents-of-histogram-to-text-file/28616/2
https://root-forum.cern.ch/t/histogram-to-ascii/14080/6