update all requests to RPC to send parameters as an object

[mollykarcher]

What problem does your feature solve?

We plan to deprecate/remove the ability to send array formatted request parameters to the RPC endpoints. See conversation here stellar/soroban-rpc#13

What would you like to see?

The SDK formats all requests as objects, to avoid any issues when the RPC stops accepting array-based parameters. We'll also need to communicate this change to all external SDK developers.