Create the getLedgers endpoint

[L-Zuluaga]

ℹ️ This issue is part of Epic #158

Objective

It’s required to create the getLedgers endpoint in the soroban.ex repo. The info about the endpoint specification can be found in the soroban-rpc repo.

https://github.com/kommitters/soroban.ex/tree/main/lib/rpc/endpoints
stellar/stellar-rpc#111

Checklist

• Add the logic for the endpoint
• Add the respective tests
• Update the readme

[einerzg]

Currently, the getLedgers endpoint is not available.

The following issue should be followed up: stellar/stellar-rpc#111