fix: Resolve issue where NuGet package fails to automatically copy unmanaged mmdeploy.dll to executable directory in .NET Framework projects

[gitleej]

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Motivation

when use CSharp api build nuget package. if we used in .netframework project, the native dll of mmdeploy can not auto copy to target directory.

Modification

add mmdeploy.targets file in project. and modified .csproj file. after build this api project, when we used the mmdeploy.nupkg in .netframework project, it can auto copy the mmdeploy.dll to generate target directory.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

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