Expose account allowances, reward payouts, and airdrop Mirror Node endpoints #100

Summary

AccountRepository currently only exposes account detail lookup, while the Mirror Node accounts REST surface also includes:

• GET /api/v1/accounts/{id}/allowances/crypto
• GET /api/v1/accounts/{id}/allowances/tokens
• GET /api/v1/accounts/{id}/allowances/nfts
• GET /api/v1/accounts/{id}/rewards
• GET /api/v1/accounts/{id}/airdrops/outstanding
• GET /api/v1/accounts/{id}/airdrops/pending

Current gap

The library already parses pending_reward into AccountInfo, but it has no public query surface or data models for:

• allowance pages
• staking reward payout history
• pending/outstanding airdrops

Proposed implementation

• Extend MirrorNodeClient / AccountRepository for the missing account-centric endpoints
• Add typed models for crypto allowances, token allowances, nft allowances, staking reward payouts, and airdrops
• Add Spring + MicroProfile parity and tests

NOTE: In progress — implementation is nearly complete. I'll open a PR soon

Acceptance criteria

• All documented account allowance/reward/airdrop endpoints are reachable
• Responses are mapped to typed models
• Spring and MicroProfile remain feature-parity