Downloading of pre-rendered plots

[nuest]

The timeseries api also supports server-side rendering. This is not core feature of the package but could be nice to have if a user wants to download a lot of data and check roughly how it looks beforehand. Some ideas:

# the following are POST requests!
getData(t, content = "image/png or application/pdf", file = .., style = list())
# stores image at the given location, or in workspace

# query rendedred timeseries with style and other rendeing options - minimal support only!
getData(t, ..., renderingOptions = list(style = '{ "chartType": "line", ...}', legend = TRUE, generalize = TRUE, format = .., ))

Check if it is easily possible to open the image from R (without having too many dependencies in the package).