Add Mirror Node support for global balances and network address book nodes

[alejandroGM0]

Summary

Current balance/network coverage is incomplete.

Today the library supports:

• token balances via /api/v1/tokens/{id}/balances
• network exchange rates, fees, stake, and supply

It does not support:

• GET /api/v1/balances
• GET /api/v1/network/nodes

Current gap

There is no repository/client entry point for the global balances API and no model for the network node address book response.

The existing Balance model is token-balance-oriented and does not match the nested /api/v1/balances payload, which includes a snapshot timestamp and optional per-account token balances.

Proposed implementation

• Add public support for /api/v1/balances
• Add public support for /api/v1/network/nodes
• Introduce typed models for:
  • balance snapshot / account balance entry / nested token balance entry
  • network node / service endpoint / staking fields
• Add tests for paging and parsing

Acceptance criteria

• Global balances and network nodes are queryable through the library
• Typed models cover the documented payloads
• Spring and MicroProfile stay aligned