Validate: add support for kubernetes deployment validation with `src validate kube` subcommand

Created by: jdpleiness

This PR adds the src validate kube subcommand for validating Sourcegraph deployments on Kubernetes.

Features include:

• pod validation
• service validation
• PVC validation
• network connection validation between Sourcegraph services

Validations can also include warnings for non-failure states that should be addressed.

Kubernetes cluster configuration is handled via standard kubeconfig file in users home directory or can be specified with --kubeconfig flag. Users can also specify a specific namespace via --namespace flag.

TODO:

• Add --quiet flag to silence command line output in CI/CD pipelines
• Update CHANGELOG.md

Test plan

• Locally tested changes using latest Sourcegraph Helm install.
• Added unit tests in internal/validate/kube/kube_test.go.
• ran go test ./... - all tests passing
• ran staticcheck ./... - no issues found in new code