roundrobin: Delegate subchannel creation to pickfirst

[arjan-bal]

This PR switches round round robin to defer to pick first instead of creating and handling SubConns itself. This is part of DualStack, and makes this a petiole. This PR makes roundrobin a wrapper around the endpoint sharding policy that implements the roundrobin logic.

RELEASE NOTES:

• balancer/roundrobin: Round Robin uses pick first child policies to manage endpoints instead of creating SubConns.
• balancer/roundrobin: Round Robin uses the Endpoints field instead of the Addresses field in resolver state updates.

[codecov]

Codecov Report

Attention: Patch coverage is 87.03704% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.99%. Comparing base (724f450) to head (3a594f6).

Files with missing lines	Patch %	Lines
balancer/roundrobin/roundrobin.go	78.12%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7966      +/-   ##
==========================================
- Coverage   82.05%   81.99%   -0.06%     
==========================================
  Files         381      381              
  Lines       38539    38558      +19     
==========================================
- Hits        31622    31616       -6     
- Misses       5602     5633      +31     
+ Partials     1315     1309       -6     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	87.25% <100.00%> (-0.03%)	⬇️
balancer/weightedroundrobin/balancer.go	82.92% <100.00%> (-0.79%)	⬇️
internal/testutils/balancer.go	87.11% <100.00%> (+4.76%)	⬆️
internal/testutils/wrr.go	91.30% <100.00%> (+1.30%)	⬆️
balancer/roundrobin/roundrobin.go	77.14% <78.12%> (-22.86%)	⬇️

... and 26 files with indirect coverage changes