Add support for the lapply backend operation

[mihaiconstantin]

This PR adds support for the lapply backend operation as requested in #30.

[codecov]

Codecov Report

Merging #31 (3183e83) into main (3fdba7e) will decrease coverage by 3.75%.
The diff coverage is 40.00%.

❗ Current head 3183e83 differs from pull request most recent head e02619e. Consider uploading reports for the commit e02619e to get more accurate results

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   94.94%   91.20%   -3.75%     
==========================================
  Files          17       17              
  Lines         356      375      +19     
==========================================
+ Hits          338      342       +4     
- Misses         18       33      +15     

Impacted Files	Coverage Δ
R/AsyncBackend.R	87.50% <0.00%> (-6.89%)	⬇️
R/Context.R	92.30% <0.00%> (-7.70%)	⬇️
R/Service.R	90.00% <0.00%> (-10.00%)	⬇️
R/SyncBackend.R	89.18% <0.00%> (-5.10%)	⬇️
R/ProgressTrackingContext.R	90.19% <71.42%> (-7.48%)	⬇️