Allows specifying `api_version` as default instead of specifying `api_version` for each request

[tequdev]

The following processes that are possible in xrpl.js should also be possible in xrpl-py.

import { Client } from 'xrpl'
const client = new Client('custom-api-v1-server')
client.apiVersion = 1

await client.connect() // api_version = 1

await client.getServerInfo()  // api_version = 1
await client.request({ comand: "server_info" }) // api_version = 1