GitLab

* Add WireTap interface

WireTap interface can be used to access all messages send and received
by Bitswap. This can be used to implement advanced statistics/analysis
logic, which is beyond scope of Bitswap, but can be implemented as IPFS
plugin.

Some examples of potential applications:
 - per CID bandwidth tracker (see: https://gitcoin.co/issue/PinataCloud/apollo/2/100023631)
 - detailed per peer stats
 - intrusion detection system (IDS) implementation

* Add test for WireTap