refactor(gossipsub): use send_message for all RpcOut

[elenaf9]

Description

Follow-up on libp2p#5595 (comment).

I've played around a bit with restoring the old Behavior::send_message function.
It is now using a new RpcSender::send_message method, that handles sending of all Rpc messages, instead of having separate methods RpcSender::{publish, subscribe, forward, ...}.
The advantages of this design IMO are:

1. Smaller diff to the current master branch
2. less code
3. Behavior::send_message handles all errors in sending. Before, there was quite a bit of code duplication for a) handling that the target peer was not among the connected peers, and b) handling if a channel is full (logging the error, increasing the counter) c) handling that the channel will never be full for high priority control messages.
4. single match statement in RpcSender::send_message that decides which rpc message is sent on which channel,

Drawbacks:

1. RpcSender::send_message returns an Result for all rpc message, even though there are some high priority messages where sending can never fail. Behavior::send_message handles a returned error for these variants with an unreachable, but obviously it would be nicer if that logic would be directly inside the RpcSender.

Notes & open questions

• We could theoretically now also restore the ControlMessagePool logic, but based on an out of band discussion with @jxs, this logic is not needed/ wanted anymore.
• feat(gossipsub): introduce backpressure libp2p/rust-libp2p#5595 (comment):

I would expect a types module to not implement any major logic apart from converting and formatting.

Wdyt of moving the RpcSender and RpcReceiver into a separate module behavior/rpc.rs or similar?

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Thanks for taking the time to test the solution Elena, and thanks for other parts of code that were also not elegant.
Looks good to me! Left just a couple comments

[jxs]

Thanks Elena! Going to merge this and address your remaining comments on libp2p#5595