Enhance traffic observability by using SendEnvelope instead of Send

[staheri14]

Context

When you want to send data to another peer, two methods can be used: Send (link) and SendEnvelope (link). Note that SendEnvelope actually uses Send internally.
If we look at the code, we see that Send is used directly in two places: in the CAT mempool and in the Broadcast method (link) of switch. However, it seems like switch's Broadcast isn't used anywhere else, and instead, the code prefers to use BroadcastEnvelope everywhere else. Considering this, it appears that SendEnvelope is the preferred choice in most cases.
Sticking to the usage of one method, e.g., SendEnvelope could enhance the clarity of monitoring outgoing traffic (note that it is preferred to monitor the message type as well which is available in the SendEnvelope side, before marshalling the message to bytes).

Problem

With the current state of the celestia-core, we lack observability into the messages (and message types) exchanged via the CAT mempool Reactor, using P2P Prometheus metrics (MessageReceiveBytesTotal and MessageSendBytesTotal). To provide that visibility, we need to utilize use SendEnvelope in that reactor instead of Send. This change should also be applied to other parts of the code that might be using Send.