pickfirst: Register a health listener when used as a leaf policy

[arjan-bal]

As part of the dualstack changes described in A61, pickfirst will become the universal leaf policy. This PR provides a EnableHealthListener function for petiole policies to inform pickfirst when it's functioning as a leaf policy.

When functioning as a leaf policy, pickfirst will subscribe to health updates using the SubConn.RegisterHealthListener API introduced in #7780 once the SubConn connectivity state becomes READY. The health state will be used to update the ClientConn state as long as the SubConn's connectivity state remains READY.

RELEASE NOTES: N/A

[codecov]

Codecov Report

Attention: Patch coverage is 84.14634% with 13 lines in your changes missing coverage. Please review.

Project coverage is 81.78%. Comparing base (274830d) to head (a1517c7).

Files with missing lines	Patch %	Lines
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	84.14%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7832      +/-   ##
==========================================
- Coverage   81.90%   81.78%   -0.12%     
==========================================
  Files         374      374              
  Lines       37978    38029      +51     
==========================================
- Hits        31105    31103       -2     
- Misses       5579     5614      +35     
- Partials     1294     1312      +18     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	87.75% <84.14%> (-1.09%)	⬇️

... and 21 files with indirect coverage changes