leastrequest: Delegate subchannel creation to pickfirst

[arjan-bal]

This PR switches the least request balancer to defer to pick first instead of creating and handling SubConns itself. This is part of Dualstack, and makes this a petiole.

RELEASE NOTES:

• balancer/leastrequest: Add support for Dualstack.

[codecov]

Codecov Report

Attention: Patch coverage is 89.02439% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.01%. Comparing base (f35fb34) to head (dff9584).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/leastrequest/leastrequest.go	89.02%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7969      +/-   ##
==========================================
- Coverage   82.17%   82.01%   -0.16%     
==========================================
  Files         381      381              
  Lines       38546    38581      +35     
==========================================
- Hits        31676    31644      -32     
- Misses       5560     5612      +52     
- Partials     1310     1325      +15     

Files with missing lines	Coverage Δ
balancer/leastrequest/leastrequest.go	88.69% <89.02%> (+1.35%)	⬆️

... and 29 files with indirect coverage changes

[dfawley]

It seems like some new test cases should be needed, in light of the changes here?