Update for new `flow.json` schema

Flow CLI version v.0.25.1 changed plural keys to singular key on service account, breaking the framework.
init method will now properly source it's value from key field and provides a fallback in case similar case occur in the future.
port and pkey can now be passed via options object as second argument:

const port = 8080;
const pkey = "48a1f554aeebf6bf9fe0d7b5b79d080700b073ee77909973ea0b2f6fbc902"

await init(basePath, { port, pkey });

Both of them are optional, so if you are OK with default values, you can make it into one-liner:

await init(basePath);