Add getPayerTotal helper method

[MarcusDavidG]

The contract exposes get_payer_total(invoice_id, payer) returning the total amount paid by a specific address. Add this as a read-only method on SharpyClient.

[MarcusDavidG]

affa I don assign you this issue.

sdk — Add getPayerTotal helper method: #18

fork the repo first before making any changes, then open a PR referencing the issue number.

Org: https://github.com/stellar-sharpy
SDK repo: https://github.com/stellar-sharpy/sharpy-sdk
Contributing guide: https://github.com/stellar-sharpy/sharpy-sdk/blob/main/CONTRIBUTING.md

hit me up if you need any info but everything dey here