cluster: support ng-monitoring config via server_configs and per-instance overrides

[boltandrke]

What problem does this PR solve

Adds support for configuring ng-monitoring via topology YAML, addressing
the gap described in #2325.

Currently there is no way to set ng-monitoring configuration (e.g.
storage.type, storage.path, continuous_profiling.*) through
the topology YAML. The NgMonitoringConfig template is hardcoded with
no user-config merge step, unlike other components (PD/TiKV/TiDB) which
support server_configs + per-instance config overrides.

What is changed and how it works

1. ServerConfigs.NGMonitoring — added ng_monitoring map[string]any to ServerConfigs for global ng-monitoring config
2. PrometheusSpec.NgMonitoringConfig — added ng_monitoring_config map[string]any for per-instance overrides
3. MonitorInstance.InitConfig — replaced template-based ngmonitoring.toml generation with map-based approach using Merge2Toml, following the same pattern as PD/TiKV/TiDB
4. Example topology — updated with documented ng_monitoring and ng_monitoring_config sections

Usage

server_configs:
  ng_monitoring:
    storage.type: "sqlite"
    storage.path: "/tidb-data/prometheus-9090/docdb_sqlite"
    continuous_profiling.enable: true
    continuous_profiling.profile_seconds: 5

monitoring_servers:
  - host: 10.0.1.21
    ng_port: 12020
    ng_monitoring_config:
      continuous_profiling.data_retention_seconds: 259200

Check List

• Code compiles (go build ./...)
• All existing tests pass (go test ./pkg/cluster/spec/...)
• New test added (TestNGMonitoringServerConfig)
• Lint passes (make check — 0 issues)

Ref #2325

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[ti-chi-bot]

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