feat: errorExpression for http steps (#6356)

[zoeyfyi]

Description

Closes #6356

Adds errorExpression to a http steps config. If a http step reaches a terminal failure (i.e. failureExpression evaluates to true) we attempt to extract an error message from the response by evaluating errorExpression. If evaluation fails, fall back to the original error message, otherwise we add the extracted error message to the message to the promotion step result.

Checklist

Eligibility

• Linked to an existing issue with no blocking labels (kind/proposal, needs discussion, needs research, maintainer only, area/security, size/large, size/x-large, size/xx-large).
• Changes documentation only.
• Changes ten lines or fewer.

Quality

• Adds or updates corresponding tests.
• Adds or updates corresponding documentation.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.36%. Comparing base (edc4a3c) to head (dd5e2d5).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6414      +/-   ##
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+ Coverage   58.30%   58.36%   +0.06%     
==========================================
  Files         500      500              
  Lines       41920    42018      +98     
==========================================
+ Hits        24441    24525      +84     
- Misses      16011    16024      +13     
- Partials     1468     1469       +1     

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[krancour]

Thank you @zoeyfyi! I only looked at docs so far -- which look good overall. My only ask would be to see if this feature can be worked into an example. As you had called out in the original issue, expr lang features like nil coalescing play a particularly important role here. Or expressions that check response code first to determine if a body from which to extract information may even exist may also be realistic.

@fuskovic would you mind reviewing the implementation please?