feat: Setup Horizon API Client with reqwest

[Oluwaseyi89]

Description

This PR implements a typed Horizon API client using reqwest to query the Stellar network. The client provides methods for fetching account details, transactions, payments, and individual transaction data with proper error handling.

Changes Made

• Added reqwest, serde, serde_json, tokio, and thiserror to dependencies
• Created src/horizon/client.rs with HorizonClient struct and implementation
• Added src/horizon/mod.rs to expose the module
• Defined comprehensive response structs with serde::Deserialize
• Implemented custom HorizonError enum for typed error handling
• Added convenience constructor for public testnet access

Implemented Methods

• get_account(address: &str) -> Result<AccountResponse, HorizonError>
• get_transactions(address: &str, cursor: Option<&str>) -> Result<TransactionPage, HorizonError>
• get_payments(address: &str, cursor: Option<&str>) -> Result<PaymentPage, HorizonError>
• get_transaction(hash: &str) -> Result<TransactionDetail, HorizonError>

Acceptance Criteria Met

• Horizon client can fetch account details
• Horizon client can fetch transactions
• Errors are properly typed and handled

Closes #8

[ayshadogo]

@Oluwaseyi89 please resolve conflicts

[Oluwaseyi89]

@ayshadogo conflict has been resolved and all checks have passed. Please attend to the PR.