Add Connection() method to the Client interface

[matthias-relabs]

Connection() returns raw net.Conn network connection. This makes it possible for plugins to access TLS credentials of a Client (by casting to a *tls.Conn), or to associate clients with the a connection from an OnAccept callback (which does not have a Client yet).

The use-case is like this: I'm using mutual tls (tls.RequireAndVerifyClientCert). Additionally I've implemented a plugin which uses information from the client certificate to chose a client policy (i.e. the topics a client is permitted to subscribe and/or publish to).

[codecov-io]

Codecov Report

Merging #21 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   70.07%   70.06%   -0.02%     
==========================================
  Files          26       26              
  Lines        2697     2699       +2     
==========================================
+ Hits         1890     1891       +1     
- Misses        642      644       +2     
+ Partials      165      164       -1

Impacted Files	Coverage Δ
client.go	82.26% <0%> (+0.07%)	⬆️
server.go	48.23% <0%> (-0.18%)	⬇️

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