Add summary stat plot for categorical concepts

[milanmlft]

Follow-up from #12 and #32, realised we didn't have the plot yet for categorical concepts 🙃

Using some dummy date, the plot should look like this:

  mock_stats <- data.frame(
    concept_id = rep(1234567, 3),
    summary_attribute = rep("frequency", 3),
    value_as_string = paste0("cat_", seq(3)),
    value_as_number = c(42, 23, 68)
  )

summary_stat_plot(mock_stats, plot_title = "test")

I chose to order the bars by frequency (which is why cat_3 is appearing as the first), but that's just an aesthetic detail, happy to change :)

[stefpiatek]

Looks good,thanks Milan