Subscribe only to specific publishers

[thedevbirb]

From sub/socket.rs:

    /// Subscribes to the given topic. This will subscribe to all connected publishers.
    /// If the topic does not exist on a publisher, this will not return any data.
    /// Any publishers that are connected after this call will also be subscribed to.
    pub async fn subscribe(&mut self, topic: impl Into<String>) -> Result<(), SubError> {

So if a consumer of this library wants to have hybrid setups where it only wants to subscribe to a subset of publishers, it needs to create multiple sub-sockets and potentially merging the streams into one.

As such, we may offer a more specific version of this function:

    /// Subscribes to the given topic. This will subscribe only to the provided publisher.
    /// If the topic does not exist on a publisher, this will not return any data.
    pub async fn subscribe_to_publishers(&mut self, topic: impl Into<String>, publishers: Vec<SocketAddr>) -> Result<(), SubError> {

I want to create a discussion first, I'm still on the fence whether we actually need this.