Add ability to set a custom high-priority transport in NewDefaultServer

Thank you for creating such a great library.

This is just a minor suggestion.

### What happened?

NewDefaultServer is a convenient function, but when I set my own transport in the return value, it gets added last, so the priority becomes low.

### What did you expect?

When setting a higher priority, I have to copy the entire code. So, it would be convenient if I could set it in an argument or something when setting a higher priority.

### versions
- `go run github.com/99designs/gqlgen version`?
v0.17.24

- `go version`?

go version go1.19.3 darwin/arm64