Support viewing Child nodes through a Netdata Parent

[arjunkomath]

Context

A Netdata Parent can receive and store metrics streamed from multiple Child Agents. The app currently treats every configured URL as a single Agent and only displays the default host exposed by that URL.

This is the modern interpretation of the request in #39. Netdata now uses Parent/Child terminology; the legacy Registry is not required for this feature.

Goal

Allow users who add a Netdata Parent to discover its available nodes, select a Child, and view that Child's metrics through the Parent without connecting directly to the Child.

Proposed behavior

• Continue supporting direct connections to standalone Agents without additional setup.
• Detect or query available nodes from a Parent using /api/v3/nodes.
• Present the Parent and available Children in a node picker.
• Route info, chart, data, and alarm requests through the selected node where supported, using the Parent's /host/<hostname-or-guid>/api/... or /node/<node-id>/api/... routes.
• Preserve the configured Parent URL, base path, authentication, and query parameters.
• Handle Parents that do not expose node discovery with a clear fallback to the existing single-Agent experience.

Acceptance criteria

• A standalone Agent behaves exactly as it does today.
• A Parent lists the nodes whose metrics it stores.
• Selecting a Child displays that Child's detail metrics and charts.
• Switching nodes cancels the previous refresh before requesting the new node.
• Missing, offline, or unauthorized Children show an actionable error without affecting other nodes.
• The implementation does not depend on the legacy Netdata Registry or Netdata Cloud.

References

• Original request: Feature Request  #39
• Netdata node discovery: /api/v3/nodes
• Legacy-compatible host routing: /host/<host>/api/v1/... and /node/<node-id>/api/v1/...