Create the getTransactions endpoint

[L-Zuluaga]

ℹ️ This issue is part of Epic #158

Objective

It’s required to create the getTransactions endpoint in the soroban.ex repo. The info about the endpoint specification can be found in the stellar documentation.

https://github.com/kommitters/soroban.ex/tree/main/lib/rpc/endpoints
https://developers.stellar.org/docs/data/rpc/api-reference/methods/getTransactions

Checklist

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