pickfirst: Interleave IPv6 and IPv4 addresses for happy eyeballs

[arjan-bal]

This PR adds the address interleaving mentioned in A61

In the flattened list, interleave addresses from the two address families, as per RFC-8305 section 4. Doing this on the flattened address list ensures the best behavior if only one of the two address families is working.

RELEASE NOTES:

• The experimental pickfirstleaf LB policy interleaves IPv4 and IPv6 address as described in RFC-8305 section 4 before starting connection attempts.

[codecov]

Codecov Report

Attention: Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (ad81c20) to head (7b42222).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7742      +/-   ##
==========================================
- Coverage   82.07%   79.84%   -2.24%     
==========================================
  Files         362      365       +3     
  Lines       28116    36409    +8293     
==========================================
+ Hits        23075    29069    +5994     
- Misses       3839     6153    +2314     
+ Partials     1202     1187      -15     

Files with missing lines	Coverage Δ
internal/testutils/balancer.go	84.40% <100.00%> (+0.97%)	⬆️
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	88.83% <97.95%> (-0.06%)	⬇️

... and 317 files with indirect coverage changes