[Statmodel] Enable user to download response curve plot

[tonywu1999]

Story

As of now, users can download a zipfile of other plots, but not the dose response curve plots yet.

Currently, whenever a user clicks to plot a visualization on the GUI, the GUI will create a visualization file with a prefix Ex_, e.g. Ex_volcanoplot.zip here. And then afterwards, it will take the most recently written file that has that Ex_ prefix and use that zipfile as the file that the user downloads here

Subtasks

• Scope the visualizeResponseProtein function
  • Add a new parameter address that is a string of a filepath to save the plot. If set to FALSE, then don't save it anywhere. If set to NULL, then save it to the current directory.
• In create_download_plot_handler, refactor the code s.t. when downloading a plot, MSstatsShiny calls code that generates the plot and its zipfile, gets a reference to the zipfile stored as a variable, and then uses that reference to place the zipfile in the user's download folder.
  • Do this for only dose response curve plots for now as a proof of concept. Create an issue to adjust this for the other plots in the future.