[REQ] New ASP.NET Core 3.0 server generator

To start with, we'll need to create the following:

• new AspNetCore3ServerCodegen.java based on AspNetCoreServerCodegen.java
• new resource folder modules/openapi-generator/src/main/resources/aspnetcore3 based on modules/openapi-generator/src/main/resources/aspnetcore (2.0 or 2.1)
• add batch file to generate Petstore samples under samples/server/petstore/aspnetcore3

cc @SeanFarrow