feat: expose pending claimable balance

[drohit-cb]

What changed? Why?

This PR helps expose the pending_claimable_balance at SDK level that was being returned from the backend starting this PR #48

Testing

Example show stakeable, unstakeable and pending claimable balances of a wallet.

>> go run examples/ethereum/shared-eth-stake/main.go /Users/rohit/code/cb/staking/.coinbase_cloud_api_key_production.json 0x87Bf57c3d7B211a100ee4d00dee08435130A62fA
2025/02/10 15:24:13 stakeable balance: Balance { amount: '3.254773242' asset: 'Asset { networkId: 'ethereum-holesky' assetId: 'eth' contractAddress: '' decimals: '18' }' }
2025/02/10 15:24:13 unstakeableBalance balance: Balance { amount: '0.2119132771' asset: 'Asset { networkId: 'ethereum-holesky' assetId: 'eth' contractAddress: '' decimals: '18' }' }
2025/02/10 15:24:13 pending claimable balance: Balance { amount: '9.8e-17' asset: 'Asset { networkId: 'ethereum-holesky' assetId: 'eth' contractAddress: '' decimals: '18' }' }

Qualified Impact

[cb-heimdall]

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