Move Property Assignment Function into the functions package

[theunrepentantgeek]

What this PR does / why we need it:

Moves PropertyAssignmentFunction into the functions package so that it's more discoverable and reusable. Required removing some functionality from the function so that it could stay in the conversions package to avoid circular references.

Reuse of this function is required by WIP to create a ConvertResourceFunction, necessary for implementing the Convertible interface.

Harvested from PR #1533

Prerequisites

• Merge PR Persist original ARM API version for use by generic controller #1606
• Merge PR Convert Direction from enum into a discriminated union #1618
• Rebase on master

How does this PR make you feel:

If applicable:

• this PR contains tests

[codecov-commenter]

Codecov Report

Merging #1622 (4531b89) into master (5e6b16b) will increase coverage by 0.00%.
The diff coverage is 89.79%.

@@           Coverage Diff           @@
##           master    #1622   +/-   ##
=======================================
  Coverage   64.88%   64.89%           
=======================================
  Files         203      203           
  Lines       12603    12614   +11     
=======================================
+ Hits         8178     8186    +8     
- Misses       3711     3715    +4     
+ Partials      714      713    -1     

Impacted Files	Coverage Δ
hack/generator/pkg/astbuilder/builder.go	82.44% <0.00%> (-2.26%)	⬇️
...n/pipeline/inject_property_assignment_functions.go	66.66% <100.00%> (ø)
.../generator/pkg/conversions/property_conversions.go	78.89% <100.00%> (+0.21%)	⬆️
...or/pkg/conversions/readable_conversion_endpoint.go	96.66% <100.00%> (+3.33%)	⬆️
...or/pkg/conversions/writable_conversion_endpoint.go	95.23% <100.00%> (+3.57%)	⬆️
...ator/pkg/functions/property_assignment_function.go	76.98% <100.00%> (ø)
hack/generated/pkg/genruntime/resolver.go	84.74% <0.00%> (+3.38%)	⬆️

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