apmbench: Support sending the benchmark data in a random order

## Description

`apmbench` currently sends the data in the same order as it's loaded, while this works fine for now, it would be interesing to add support to the `benchtest/eventhandler` package to allow shuffling the data when the slice is iterated over.

## Possible solution

We could add a `-shuffle` flag to `apmbench` which may use [`math/rand.Perm`](https://pkg.go.dev/math/rand#Rand.Perm) to randomize the access to the slice of readers that is used to send the data to the APM Server.