Add show_omitted for "spcv_buffer"

[pat-s]

library(mlr3)
library(mlr3spatiotempcv)
task <- tsk("ecuador")
resampling <- rsmp("spcv_buffer", theRange = 1000)
resampling$instantiate(task)

## single fold
autoplot(resampling, task, fold_id = 1, show_omitted = TRUE) +
  ggplot2::scale_x_continuous(breaks = seq(-79.085, -79.055, 0.01))

^{Created on 2023-08-15 with reprex v2.0.2}

[codecov-commenter]

Codecov Report

Merging #228 (4667620) into main (120e01b) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

❗ Current head 4667620 differs from pull request most recent head 2f222bd. Consider uploading reports for the commit 2f222bd to get more accurate results

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
- Coverage   79.00%   78.97%   -0.04%     
==========================================
  Files          29       29              
  Lines        2020     2021       +1     
==========================================
  Hits         1596     1596              
- Misses        424      425       +1     

Files Changed	Coverage Δ
R/helper_autoplot.R	88.37% <0.00%> (-2.11%)	⬇️
R/autoplot.R	42.12% <100.00%> (ø)