feat(templates): templating functions

[james-d-elliott]

This adds several functions which are available in most areas that use templates.

[authelia]

Artifacts

These changes are published for testing on Buildkite, DockerHub and GitHub Container Registry.

Docker Container

• docker pull authelia/authelia:feat-template-funcs
• docker pull ghcr.io/authelia/authelia:feat-template-funcs

[codecov]

Codecov Report

Merging #4635 (40630e6) into master (a916b65) will increase coverage by 0.15%.
The diff coverage is 77.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4635      +/-   ##
==========================================
+ Coverage   67.31%   67.47%   +0.15%     
==========================================
  Files         265      265              
  Lines       18876    18973      +97     
  Branches      431      431              
==========================================
+ Hits        12707    12802      +95     
- Misses       5427     5429       +2     
  Partials      742      742              

Flag	Coverage Δ
backend	67.89% <77.30%> (+0.17%)	⬆️
frontend	62.65% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rnal/configuration/koanf_provider_filtered_file.go	73.41% <50.00%> (-3.59%)	⬇️
internal/templates/funcs.go	67.33% <76.61%> (+67.33%)	⬆️
internal/templates/util.go	42.46% <86.66%> (+12.46%)	⬆️

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