gracefulswitch: add ParseConfig and make UpdateClientConnState call SwitchTo if needed

[dfawley]

cc @anicr7

This change does not migrate some users of gracefulswitch; they can be done as a follow-up: clusterimpl, outlierdetection, balancergroup.

RELEASE NOTES: none

[codecov]

Codecov Report

Merging #7035 (d3d4605) into master (dadbbfa) will decrease coverage by 0.14%.
The diff coverage is 84.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7035      +/-   ##
==========================================
- Coverage   82.45%   82.31%   -0.14%     
==========================================
  Files         299      300       +1     
  Lines       31314    31349      +35     
==========================================
- Hits        25819    25805      -14     
- Misses       4435     4471      +36     
- Partials     1060     1073      +13     

Files	Coverage Δ
balancer/balancer.go	96.00% <100.00%> (+0.16%)	⬆️
balancer_wrapper.go	83.51% <100.00%> (+0.58%)	⬆️
clientconn.go	90.76% <100.00%> (-2.00%)	⬇️
pickfirst.go	82.22% <100.00%> (-0.52%)	⬇️
service_config.go	79.04% <84.21%> (+0.09%)	⬆️
internal/balancer/gracefulswitch/gracefulswitch.go	88.79% <82.60%> (-0.88%)	⬇️
internal/balancer/gracefulswitch/config.go	80.00% <80.00%> (ø)

... and 13 files with indirect coverage changes

[zasweq]

This change LGTM, what was the technical reason this came up (other than the fact this is now how Java and C does it)? Will let Arvind gate the merge though since he's official reviewer.

[dfawley]

Comments addressed

This change LGTM, what was the technical reason this came up (other than the fact this is now how Java and C does it)? Will let Arvind gate the merge though since he's official reviewer.

The reason we did this is that we had a user that wanted to delegate to a child using GSB but didn't want to have to do all the parsing logic.