Custom Tester - similar to custom builder

### Expected behavior

A custom tester for container structure tests can be defined similar to a custom builder.

This would be especially handy when triggering builds and deployments on architecture A that are built / run remotely on architecture B.

### Actual behavior

The tester requires
a) the container-structure-test binary to be installed locally
b) the test to be executed / container to run locally

### Information

Similar to `artifacts[].custom.buildCommand` `test[|.custom.testCommand` should be configurable with a contract defined similar to custom builders.

To get a better idea for the need - have a look at https://github.com/helmuthva/jetson/blob/master/workflow/deploy/tools/builder which is how I use Skaffold to trigger builds and K8s deploys on a macOS device that are in effect executed remotely on arm64 based NVIDIA Jetson edge devices integrated in a multiarch K8s cluster.

### Steps to reproduce the behavior

`skaffold build --profile quickcheck`

results in

`test failed: running structure tests: running container-structure-test: exec: "container-structure-test": executable file not found in $PATH`

as the test should run on a remote device.

Btw: skaffold rocks.