NuGet Pkg using GitHub Actions (#262)

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.

Co-authored-by: Diogo Strube de Lima <diogo.strube.lima@gmail.com>
Co-authored-by: Chris Mc <prince.chrismc@gmail.com>
Co-authored-by: Dominik Thalhammer <dominik@thalhammer.it>

Author: 3 people

.github/workflows/nuget.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: Nuget
+
+# Controls when the workflow will run
+on:
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  build:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+      # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+      - uses: actions/checkout@v3
+        with:
+          ref: 'v0.6.0'
+      
+      # Deploy NuGet so we can call the pack and push cmds
+      - name: Setup NuGet.exe for use with actions
+        uses: NuGet/setup-nuget@v1.0.7
+        with:
+          nuget-api-key: ${{ secrets.nuget_api_key }}
+          
+      # Package the nusepc file
+      - name: Create NuGet pkg
+        working-directory: ./nuget
+        run: nuget pack jwt-cpp.nuspec
+        
+      # Publish nuget pkg
+      - name: Publish NuGet pkg
+        working-directory: ./nuget
+        run: nuget push *.nupkg -Source 'https://api.nuget.org/v3/index.json'
+          

nuget/jwt-cpp.nuspec

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+<?xml version="1.0" encoding="utf-8"?>
+<package>
+  <metadata>
+    <id>jwt-cpp</id>
+    <version>0.6.0-dev0.2</version>
+    <authors>Thalhammer; prince-chrismc</authors>
+    <owners>Thalhammer; prince-chrismc</owners>
+    <projectUrl>https://github.com/Thalhammer/jwt-cpp</projectUrl>
+    <description>JWT++ is a header only library for creating and validating JSON Web Tokens in C++11. This library supports all the algorithms defined by the JWT specifications, and the modular design allows to easily add additional algorithms without any problems. In the name of flexibility and extensibility, jwt-cpp supports OpenSSL, LibreSSL, and wolfSSL. And there is no hard dependency on a JSON library.
+    </description>
+    <releaseNotes>Supporting OpenSSL 3.0.0, WolfSSL, Hunter CMake, Boost.JSON, JWKs, ES256K.</releaseNotes>
+    <license type="expression">MIT</license>
+    <copyright>Copyright (c) 2018 Dominik Thalhammer</copyright>
+    <title>JWT++: JSON Web Tokens in C++11</title>
+    <summary>JWT++; a header only library for creating and validating JSON Web Tokens in C++11.</summary>
+    <tags>JWT, json, web, token, C++, header-only</tags>
+    <dependencies>
+      <group targetFramework="native0.0">
+	    <dependency id="PicoJSON" version="1.3.0" />
+	    <dependency id="openssl-vc141-static-x86_64" version="1.1.0" />
+	  </group>
+    </dependencies>
+  </metadata>
+  <files>
+    <file src="..\include\jwt-cpp\**" target="\lib\native\include\jwt-cpp\" />
+    <file src="jwt-cpp.targets" target="\build\native\" />
+    <file src="..\README.md" target="docs\" />
+  </files>
+</package>

nuget/jwt-cpp.targets

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>$(MSBuildThisFileDirectory)..\..\lib\native\include\;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
+</Project>