add a DialAddr method to the client

[marten-seemann]

Fixes #49.

This change means that we don't have anything meaningful to return from proxiedConn.RemoteAddr, since we never learn the target's IP address. It seems like there's nothing we can do about it. The right thing to return would be the host name we dialed, but that doesn't fit into the net.PacketConn abstraction.

Once we implement support for the RFC 9209 Proxy Status headers (see #2), we can fill in the value the proxy sent. But there's no guarantee that 1. the proxy sends this header value and 2. that the value is correct.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Project coverage is 72.63%. Comparing base (ada223f) to head (2128d31).

Files	Patch %	Lines
client.go	76.47%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   71.47%   72.63%   +1.15%     
==========================================
  Files           4        4              
  Lines         284      296      +12     
==========================================
+ Hits          203      215      +12     
  Misses         57       57              
  Partials       24       24              

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