[MilCodeGen] Declare Equals/GetHashCode and property getters for fields as readonly

[h3xds1nz]

Description

Modifies MilCodeGen to generate readonly getters for fields where SkipProperties="false", in case the properties are skipped (SkipProperties="true") they live in the non-generated area and thus were modified manually.

• The Equals override uses new syntax and is readonly.
• The non-override Equals is now readonly.
• GetHashCode is readonly and all the supporting props (IsEmpty, IsDistinguishedIdentity) are readonly to prevent copies in the particular call chain.

Rest of the members that were not in the impact zone are left untouched and shall be done separately in subsequent PRs.

Customer Impact

Increased performance/better codegen.

Regression

No.

Testing

Local build, most of the generated structs have unit tests already including Equals/GetHashCode checks.

Risk

Low. The point to consider is whether we could make any of the members that are to become readonly mutate the struct in the future, for me that answer is no. Equals/GetHashCode should never modify the state, even on mutable structs.

Microsoft Reviewers: Open in CodeFlow

[h3xds1nz]

We could use [[lowerName]] (which is the resource name in lower-camelCase) if desirable instead of other.

I'm fine eitherway.

[codecov]

Codecov Report

Attention: Patch coverage is 27.77778% with 13 lines in your changes missing coverage. Please review.

Project coverage is 11.46188%. Comparing base (b6914d7) to head (46b2614).
Report is 16 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10618         +/-   ##
===================================================
+ Coverage   11.45872%   11.46188%   +0.00316%     
===================================================
  Files           3214        3214                 
  Lines         648458      648410         -48     
  Branches       71511       71511                 
===================================================
+ Hits           74305       74320         +15     
+ Misses        572989      572927         -62     
+ Partials        1164        1163          -1     

Flag	Coverage Δ
Debug	11.46188% <27.77778%> (+0.00316%)	⬆️

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