chore: harden subscribe procs in libwaku

[gabrielmer]

Background

Currently, if we call libwaku's waku_relay_subscribe N times, the message handler is set up N times and we would get N notifications per message.

We want to:

1. Allow only one message handler to be set up. i.e if waku_relay_subscribe is called when a handler is already set for that pubsub topic, then ignore the call.

2. Create a new proc in libwaku to enable subscription by content topic (for autosharding). Once that procedure is exposed, we should create a wrapper in waku-go-bindings where all the desired pubsub and content topics to subscribe are passed as arguments, and the wrapper handles the calls to libwaku to make it happen.

Please refer to the following Discord thread as reference