cluster_resolver: wait for child policy configuration update inline

[aranjans]

Problem

#5469 recommends an audit of existing LB policies to ensure that they update their pickers synchronously upon receipt of a configuration update. cluster_resolver neither does update its picker synchronously nor wait for the child policy configuration update inline.

What does this PR do?

This PR ensures that every time configuration update is triggered, it waits for child policy configuration update inline.

RELEASE NOTES:

• cluster_resolver: wait for child policy configuration update inline

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.60%. Comparing base (e524655) to head (c7e6556).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7490      +/-   ##
==========================================
+ Coverage   81.40%   81.60%   +0.20%     
==========================================
  Files         357      358       +1     
  Lines       27243    27351     +108     
==========================================
+ Hits        22176    22320     +144     
+ Misses       3850     3822      -28     
+ Partials     1217     1209       -8     

Files	Coverage Δ
...ternal/balancer/clusterresolver/clusterresolver.go	71.51% <100.00%> (+1.02%)	⬆️

... and 35 files with indirect coverage changes

[aranjans]

Closing this PR for now as we are waiting for this task to close first