Add getIconUrlFromIssuer helper

[CassioMG]

This PR adds a getIconUrlFromIssuer ported from the extension's helper with same name which should be able to fetch the asset image from issuer's TOML file if any.

In a later PR we should account for caching those images to reduce the number of requests.

This helper will work in conjunction with the Asset component to display any asset image on the app.

getIconUrlFromIssuer

Retrieves an icon URL for a Stellar asset based on its issuer information.

This function follows a multi-step process to find an asset's icon URL:

1. Queries Horizon to get the issuer account information, including home domain
2. Retrieves the stellar.toml file from the issuer's home domain
3. Searches the toml file's CURRENCIES section for matching asset code and issuer
4. Returns the image URL if found, or empty string if any step fails

The process requires multiple network requests:

• One to Horizon to get issuer info
• One to the issuer's domain to get the stellar.toml file
• Later, one to fetch the actual image (handled by the caller)

@example
// Get icon URL for USDC on testnet
const iconUrl = await getIconUrlFromIssuer({
  issuerKey: "GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5",
  assetCode: "USDC",
  networkDetails: TESTNET_NETWORK_DETAILS
});