feature: add --dry-run kubectl option

[jjo]

Fixes #666.

• add --dry-run kubectl option to apply,
  delete, prune
• add some simple testing to pkg/kubernetes/client

[jjo]

FYI this is how it looks, from a setup following https://grafana.com/blog/2021/07/12/best-practices-for-consistent-configuration-management-at-scale-with-tanka/

• apply --dry-run=server:

$ ~/go/bin/tk apply --dry-run=server environments/grafana --name grafana/kind-kind
Warning: There are no differences. Your apply may not do anything at all.
namespace/grafana unchanged (server dry run)
configmap/grafana-config unchanged (server dry run)
configmap/grafana-dashboard-provisioning unchanged (server dry run)
configmap/grafana-datasources configured (server dry run)
configmap/grafana-notification-channels configured (server dry run)
service/grafana unchanged (server dry run)
deployment.apps/grafana unchanged (server dry run)

• delete --dry-run=server:

$ ~/go/bin/tk delete --dry-run=server environments/grafana --name grafana/kind-kind
WARNING: version difference between client (1.23) and server (1.21) exceeds the supported minor version skew of +/-1
deployment.apps "grafana" deleted (server dry run)
service "grafana" deleted (server dry run)
configmap "grafana-notification-channels" deleted (server dry run)
configmap "grafana-datasources" deleted (server dry run)
configmap "grafana-dashboard-provisioning" deleted (server dry run)
configmap "grafana-config" deleted (server dry run)
namespace "grafana" deleted (server dry run)

[julienduchesne]

This looks great! Thanks for adding tests!

[inkel]

I'll leave the approval to the maintainers, as they've more context about the changes and how it affects overall, but from my end it looks good to get merged. I've left a minor comment regarding Go style.

[inkel]