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

## 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