chore: add global.json file and add sample project to solution now that it builds

StevanFreeborn.SecureConfig.slnx

@@ -1,5 +1,6 @@
 <Solution>
   <Folder Name="/src/">
+    <Project Path="src/StevanFreeborn.Extensions.Configuration.Secure.Sample/StevanFreeborn.Extensions.Configuration.Secure.Sample.csproj" />
     <Project Path="src/StevanFreeborn.Extensions.Configuration.Secure/StevanFreeborn.Extensions.Configuration.Secure.csproj" />
   </Folder>
   <Folder Name="/tests/">

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "10.0.201",
+    "rollForward": "latestFeature"
+  }
+}