Add AllowMultiple into MapEnumValueAttribute

[trejjam]

MapEnumValueAttribute is missing AttributeUsageAttribute(AllowMultiple=true)

When you want to specify multiple MapEnumValueAttribute you are not allowed by compiler

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[codecov]

Codecov Report

Merging #672 (d121867) into main (a5f6aef) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #672   +/-   ##
=======================================
  Coverage   90.97%   90.97%           
=======================================
  Files         172      172           
  Lines        6130     6130           
  Branches      780      780           
=======================================
  Hits         5577     5577           
  Misses        377      377           
  Partials      176      176           

Files Changed	Coverage Δ
...iok.Mapperly.Abstractions/MapEnumValueAttribute.cs	0.00% <ø> (ø)

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[latonz]

Thank you for this PR, lgtm 😊

[github-actions]

🎉 This PR is included in version 3.2.0-next.1 🎉

The release is available on:

• GitHub release
• v3.2.0-next.1

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.2.0 🎉

The release is available on:

• GitHub release
• v3.2.0

Your semantic-release bot 📦🚀