Fix duplication of parameter name assignment

[theunrepentantgeek]

What this PR does / why we need it:

We had the parameter name for the PropertyAssignmentFunction defined in two places, which would allow them to inadvertently move out of sync later on. This PR centralises the naming of the parameter into one place, caching the name on the function to allow easy reuse where required.

Closes #1564

How does this PR make you feel:

[codecov-commenter]

Codecov Report

Merging #1589 (de3709c) into master (3d5e5a3) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head de3709c differs from pull request most recent head 3a9e234. Consider uploading reports for the commit 3a9e234 to get more accurate results

@@            Coverage Diff             @@
##           master    #1589      +/-   ##
==========================================
- Coverage   63.25%   63.24%   -0.02%     
==========================================
  Files         183      183              
  Lines       11918    11914       -4     
==========================================
- Hits         7539     7535       -4     
  Misses       3706     3706              
  Partials      673      673              

Impacted Files	Coverage Δ
...or/pkg/conversions/property_assignment_function.go	74.01% <100.00%> (-0.58%)	⬇️

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