Add --skip-json-path to validate command

[stefanprodan]

Some manifests carry tooling-injected fields that are stripped at apply time by Flux (e.g. SOPS-encrypted Secrets). Use --skip-json-path to remove those fields from validation so the rest of the document is still checked.

Example:

flux-schema validate ./manifests \
  --skip-json-path 'v1/Secret:/sops' \
  --skip-json-path 'Deployment:/sops'

[matheuscscp]

LGTM