Unify/clean up the way to set channel options #124

What's needed?

Up until #121, all channel options could be specified via the server URL, as query parameters. Now we have an inconsistency where only the auth key and signing secret can only be passed as default channel options when instantiating the base client.

Proposed solution

The obvious solution is parse the auth key and signing secret from the server URL as other options.

Use cases

No response

Alternatives and workarounds

These being secrets, it might not be the best option, as it is usually to pass secrets as env vars, and passing secrets via the URL will convert the whole URL in a secret, which might not be necessary.

So alternatively we could split secrets as such, and separate them from options. Then we can natively read secrets from env vars too, but maybe that's not even necessary.

Additional context

No response