Tapiro generation bug when there are duplicate names for commands in different controllers

[tpetrucciani]

The *RequestPayload classes that are autogenerated by tapiro (introduced by #212) are named (CommandName)RequestPayload and are all placed in the same package.

As a result, if there are two different controllers that use the same command name (e.g. AController and BController both with @command create(...)), tapiro generates two classes CreateRequestPayload in the same package, and the code does not compile.

Solution: disambiguate the class names or, probably better, generate the classes inside object *TapirEndpoints so they are in different namespaces.

cc @stefanosambruna