Fix get_parameter for Step

[elliotgunton]

Pull Request Checklist

• Fixes Parameter Validation Error when using get_parameter with a parameter set using ValueFrom #550 for Steps
• Tests added
• Documentation/examples added
• Good commit messages and/or PR title

Description of PR

Currently, using get_parameter on a step fails due to the validator as value/value_from are mutually exclusive, e.g.

            Step(
                name="do-retry",
                template=...
                when=f"{retry_step.get_parameter('retry-param')}==retry",
            )

This PR copies the fix for Task from #565 for Step.

Only doing a small fix as the duplicated function code should be addressed in #595

[codecov]

Codecov Report

Merging #624 (72d6ae8) into main (a1d704e) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #624   +/-   ##
=====================================
  Coverage   72.3%   72.4%           
=====================================
  Files         42      42           
  Lines       2898    2897    -1     
  Branches     571     571           
=====================================
  Hits        2098    2098           
+ Misses       645     644    -1     
  Partials     155     155           

Impacted Files	Coverage Δ
src/hera/workflows/steps.py	66.0% <ø> (+0.6%)	⬆️

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