fix(gitops): support --set/--del for cluster, repository, and application (no -f required)

[BlueLabelscotch]

Summary

• Adds flags_builtin.set/del support to create/update gitops_cluster, gitops_repository, and gitops_application, so the major fields (server/connection_type, repo_url/type/connection_type, name/repo_url/path/target_revision/destination_server/namespace) can be set via --set field=value instead of requiring a -f YAML file.
• file_body on these six commands changes from required to optional — existing -f usage is unaffected (the file always wins over --set when both are given; create_body_init/--set only fill in what the file didn't provide).
• Update commands additionally support --del field via a new get-then-put merge, with update_body_pick trimmed to just the writable subset of each resource (excludes server-computed fields like connectionState/info/status/operation, which caused a 400: json: cannot unmarshal string into Go value of type map[string]json.RawMessage when naively round-tripped).