automatically set the tls.Config.ServerName if unset

[marten-seemann]

This is needed for:

1. Verifying the server certificate
2. Generating a cache key for the ClientSessionCache. If ServerName is unset, crypto/tls will discard the session ticket.

Also saves one cloning of the tls.Config when using DialAddr{Early}.

This is similar to what the standard library does: https://cs.opensource.google/go/go/+/refs/tags/go1.21.0:src/crypto/tls/tls.go;l=147-154

[codecov]

Codecov Report

Merging #4032 (022b36a) into master (bda01bc) will increase coverage by 0.09%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #4032      +/-   ##
==========================================
+ Coverage   82.85%   82.94%   +0.09%     
==========================================
  Files         147      147              
  Lines       14789    14781       -8     
==========================================
+ Hits        12253    12260       +7     
+ Misses       2036     2022      -14     
+ Partials      500      499       -1     

Files Changed	Coverage Δ
client.go	58.12% <40.00%> (+0.55%)	⬆️
transport.go	67.55% <90.91%> (+4.11%)	⬆️

... and 2 files with indirect coverage changes