wip!: use expr language for executable templates

[jahvon]

Replace Sprig and standard Go templating funcs with Expr lang to align with usage in ref config conditionals

[codecov]

Codecov Report

Attention: Patch coverage is 78.53659% with 44 lines in your changes missing coverage. Please review.

Project coverage is 38.63%. Comparing base (ee5d737) to head (1267582).

Files with missing lines	Patch %	Lines
internal/services/expr/template.go	79.25%	22 Missing and 6 partials ⚠️
internal/templates/templates.go	75.00%	3 Missing and 3 partials ⚠️
internal/services/expr/expr.go	50.00%	3 Missing ⚠️
internal/services/rest/rest.go	75.00%	3 Missing ⚠️
internal/templates/artifacts.go	50.00%	3 Missing ⚠️
internal/templates/expression.go	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
+ Coverage   37.37%   38.63%   +1.25%     
==========================================
  Files          77       79       +2     
  Lines        6579     6714     +135     
==========================================
+ Hits         2459     2594     +135     
+ Misses       3649     3643       -6     
- Partials      471      477       +6     

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