Support binding to a source address.

[miracle2k]

I want to set the source ip address for a request. I see that there is a TODO in the code. This seems to work for me, which seems almost too easy. Is there something else?

[njsmith]

There's also an open issue at #275, which has a bit more discussion of the subtleties here. This works in simple cases, but it forces a choice of IPv4/IPv6 (whichever one you use as the source), and there's some problem around picking a port given that we might be making multiple connections... let's move the discussion there.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (5ca9662) to head (f7583d7).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main         #518   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18460        18460           
  Branches         1216         1216           
===============================================
  Hits            18460        18460           

Files with missing lines	Coverage Δ
src/trio/_highlevel_ssl_helpers.py	100.00000% <ø> (ø)