feat!: use expr language for executable templates and request transformations

[jahvon]

Replaces Sprig, jq and standard Go templating funcs with Expr lang to align with usage in ref config conditionals. This update changes the templating language across Request exec transformations and all flowfile template fields. See updated docs for more info.

This is a breaking change! If these fields are set anywhere, they will need to be converted to the expr counterpart.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 82 lines in your changes missing coverage. Please review.

Project coverage is 22.65%. Comparing base (4606279) to head (165fa33).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/services/expr/template.go	79.25%	22 Missing and 6 partials ⚠️
internal/templates/templates.go	0.00%	24 Missing ⚠️
internal/templates/expression.go	0.00%	18 Missing ⚠️
internal/templates/artifacts.go	0.00%	6 Missing ⚠️
internal/services/expr/expr.go	50.00%	3 Missing ⚠️
internal/services/rest/rest.go	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
+ Coverage   21.39%   22.65%   +1.26%     
==========================================
  Files          77       79       +2     
  Lines        6604     6739     +135     
==========================================
+ Hits         1413     1527     +114     
- Misses       4923     4937      +14     
- Partials      268      275       +7     

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