NuGet Pkg for v0.6.0 using GitHub Actions

[diogo-strube]

This PR is related to discussion #243 and is a 3 file contribution:

• nuget\jwt-cpp.nuspec contains all the descriptions of the NuGet package, including authors, and the path of files to be copied.
• nuget\jwt-cpp.targets automatically set up the "additional include directories" when the package is installed.
• .github\workflows\nuget.yml contains the GitHub Action to pack and pub the NuGet package.

Some context on the current approach:

• As the version is already tracked by committed files (as shown in #208), I left the version explicit in the .nuspec file (instead of using an environment variable or secret).
• Publishing a NuGet pkg (to nugte.org) requires a token, the same was configured as a secret ${{ secrets.nuget_api_key }} in .github\workflows\nuget.yml.

[prince-chrismc]

Looks super good just a few questions

[diogo-strube]

@Thalhammer and @prince-chrismc, with the latest commit you should be ready to configure the secret and try running the workflow, it will generate a pre-release package "-dev0.2" that we can do a final validation before publishing the release package.

[Thalhammer]

@prince-chrismc Can you create a token for nuget ? I can't cause I am not a co owner of the package but you seem to be now.

[prince-chrismc]

should be in you inbox :)