Adding JSON export for actor subscription topology

[swaploard]

Allow to export actor hierarchy to JSON for inspection (#58)

This PR adds a Supervisor::to_json() export that provides a machine-readable view of actor subscription topology. This mirrors the information exposed by to_mermaid() but outputs structured JSON suitable for inspection, tooling, or testing.

[ddrcode]

Thank you for the contribution, @swaploard. I will review your code later today.

[ddrcode]

Thanks for your first PR and contribution to Maiko!

Please address the issues highlighted in the comments, as the code currently doesn't compile with

cargo build --features serde

Ensure that:

• It compiles
• The serde feature dependencies are fixed
• ActorSubscriptionExport struct is moved inside to_json method.

Thank yout!

[swaploard]

@ddrcode Thanks for the feedback!
I’ve addressed the issues:

• The code now compiles with cargo build --features serde
• Fixed the serde feature dependencies
• Moved ActorSubscriptionExport inside to_json
• Converted label() outputs into String

Please let me know if anything else needs adjustment.

[ddrcode]

Thank you for providing the changes, @swaploard - it looks good now. Approved.