fix(k8smeta): accept purely numeric Kubernetes node names

[sooraj-sky]

Summary

• Falco's downward API env var substitution infers a JSON integer type for a purely numeric value (e.g. a Kubernetes node named 123456), but the k8smeta plugin's init-config JSON schema required nodeName to be a string. Schema validation rejected the numeric value outright, so the plugin crash-looped on any node with a numeric hostname (Value type not permitted by 'type' constraint), even though the CRDs/config were otherwise valid.
• Relaxed the schema so nodeName accepts ["string", "integer"].
• In parse_init_config, an integer value is now converted back to a string (std::to_string) before being used, since the rest of the plugin (env-var substitution pattern matching, m_node_name) expects a string.

Fixes #1372

Test plan

• Added plugin_k8s_with_numeric_node_name to test/src/init_config.cpp, following the existing test pattern in that file (plugin_owner->init(...) through the real schema validator and parse_init_config).
• Built the full plugin (libk8smeta.so) and the unit-test-libsinsp test binary per this repo's CI recipe (cmake + make k8smeta + make build-tests), in an ubuntu:22.04 container matching .github/workflows/k8smeta-ci.yaml.
• Ran the init_config.cpp test group directly against the built binary — all 7 pass, including the new numeric-node-name case:

  [ RUN      ] sinsp_with_test_input.plugin_k8s_with_numeric_node_name
  [       OK ] sinsp_with_test_input.plugin_k8s_with_numeric_node_name (0 ms)
  [  PASSED  ] 7 tests.
  

  (The gRPC-server-dependent plugin_k8s_basic_API-style tests in the same binary need the Go-based mock collector server from k8s-metacollector, which wasn't built in this pass since this fix is entirely config-parsing; they're untouched by this change.)
• clang-format-14 --dry-run --Werror on both changed files — clean, matches this repo's formatting-check CI job.
• Independently validated both halves of the fix in isolation before touching the real code: the draft-04 JSON Schema change against a real validator (accepts int/string, rejects float), and the exact nlohmann::json conversion logic (numeric, string, and env-var-pattern node names all convert correctly).

[poiana]

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[poiana]

Welcome @sooraj-sky! It looks like this is your first PR to falcosecurity/plugins 🎉