Make hardcoded timeouts configurable

When attempting to reproduce some rare errors in firecracker-containerd, I've at times increased load quite a bit which has caused me to hit various hardcoded timeouts in the go sdk. When I increase the timeouts in my fork the operations succeed, so the timeouts are just set too low for the tests I'm running. The current values seem fine as defaults, but they should be configurable.

Ones I've encountered:
* https://github.com/firecracker-microvm/firecracker-go-sdk/blob/9e2ff62839b228665976be79191e33cffce5f760/machine.go#L495
* https://github.com/firecracker-microvm/firecracker-go-sdk/blob/9e2ff62839b228665976be79191e33cffce5f760/firecracker.go#L29
* https://github.com/firecracker-microvm/firecracker-go-sdk/blob/9e2ff62839b228665976be79191e33cffce5f760/firecracker.go#L173