Do not use json.RawMessage for untyped request arguments

Using `json.RawMessage` for _request_ arguments makes the users of `go-dap` unnecessary cumbersome. `any` would be a better choice. Compare

```
msg, err := json.Marshal(someArgs)
if err != nil {
....
}
AttachRequest {
Arguments: json.RawMessage(msg),
}
```
and
```
AttachRequest {
Arguments: someArgs,
}
```