pubsub/pstest: Add Reactor adapter func type

It's currently a little cumbersome to construct custom server reactors for `pstest`, you have to make a type that implements the interface which isn't a problem on it's own but can result in quite a few types needed to be defined you want to react on quite a few functions on server.

I would like to implement a simple `ReactorFunc` type, similar to the `http.HandlerFunc` type. This would be a simple adapter function that implements the `Reactor` interface.

```go
type ReactorFunc func(in any) (handled bool, ret any, err error)

func (f ReactorFunc) React(in any) (handled bool, ret any, err error) {
return f(in)
}
```

This makes it a little easier to define and use reactors:

```go
reactors := []pstest.ServerReactorOption{
{
FuncName: "Acknowledge",
Reactor: ReactorFunc(func(_ any) (handled bool, ret any, err error) {
// Do Something
return false, &emptypb.Empty{}, nil
}),
},
}

srv := pstest.Server(t, reactors...)
```

And as a bonus maybe a constructor helper:

```go
func NewServerReactorOption(fn string, reactor Reactor) pstest.ServerReactorOption {
return pstest.ServerReactorOption{
FuncName: fn,
Reactor: reactor
}
}
```