Add initial implementation of PlunkEmailSender and update .gitignore.

- Removed `appsettings.Development.json` and excluded it from version control.
- Updated `API.csproj` to remove unnecessary folder reference.

API/API.csproj

@@ -30,8 +30,4 @@
     <PackageReference Include="Ulid" Version="1.4.1" />
   </ItemGroup>
 
-  <ItemGroup>
-    <Folder Include="Services\" />
-  </ItemGroup>
-
 </Project>