Add sprig text/template functions to template stage.

[cyriltovena]

Fixes #3129

[codecov-io]

Codecov Report

Merging #3515 (6447764) into master (b06d970) will decrease coverage by 0.00%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master    #3515      +/-   ##
==========================================
- Coverage   63.05%   63.05%   -0.01%     
==========================================
  Files         206      206              
  Lines       17681    17682       +1     
==========================================
  Hits        11149    11149              
  Misses       5531     5531              
- Partials     1001     1002       +1     

Impacted Files	Coverage Δ
pkg/logentry/stages/template.go	81.03% <76.47%> (+0.33%)	⬆️
pkg/querier/queryrange/downstreamer.go	95.29% <0.00%> (-2.36%)	⬇️
pkg/querier/queryrange/limits.go	86.66% <0.00%> (-1.91%)	⬇️
pkg/logql/evaluator.go	90.19% <0.00%> (+0.35%)	⬆️
pkg/storage/stores/shipper/uploads/table.go	66.82% <0.00%> (+0.46%)	⬆️

[kavirajk]

LGTM!