CPack: Missing the 'PackageReadmeFile' property in nuget generator

nuget.org warns if a readme is not specified in the nuget package. It should look like:

<readme>my_readme_inside_the_package.md</readme>

either the implementation could be very similar to #23703 (closed) or we can consider introducing an extension point by either:

• having a way to specify your own .nuspec file instead of the hardcoded one.
• having a way to add user defined properties the "metadata" section

This way we do not have to support every property ever in cpack but instead let projects roll their own.

references:

• https://learn.microsoft.com/en-us/nuget/create-packages/package-authoring-best-practices#readme