client: Change connectivity state to CONNECTING when creating the name resolver

[easwars]

Fixes #7686

Current Behavior

• When client exits IDLE and creates the name resolver, it stays in IDLE until the connectivity state is set by the LB policy.
• When exiting IDLE mode (because of Connect being called or because of an RPC), if name resolver creation fails, we stay in IDLE.

New Behavior

• When the client exits IDLE and creates the name resolver, it moves to CONNECTING. Moving forward, the connectivity state will be set by the LB policy.
• When exiting IDLE mode (because of Connect being called or because of an RPC), we have already moved to CONNECTING (because of the previous bullet point). If name resolver creation fails, we will move to TRANSIENT_FAILURE and start the idle timer and move back to IDLE when the timer fires

RELEASE NOTES:

• client: Change connectivity state to CONNECTING when creating the name resolver (as part of exiting IDLE).
• client: Change connectivity state to TRANSIENT_FAILURE if name resolver creation fails.
• client: Change connectivity state to IDLE after idle timeout expires (also when current state is TRANSIENT_FAILURE).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.27%. Comparing base (112ec12) to head (86503d2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8710      +/-   ##
==========================================
- Coverage   83.28%   83.27%   -0.01%     
==========================================
  Files         416      416              
  Lines       32267    32272       +5     
==========================================
+ Hits        26874    26875       +1     
- Misses       4019     4024       +5     
+ Partials     1374     1373       -1     

Files with missing lines	Coverage Δ
clientconn.go	90.50% <100.00%> (+0.36%)	⬆️
internal/idle/idle.go	89.28% <100.00%> (+0.12%)	⬆️
resolver_wrapper.go	92.72% <100.00%> (+0.27%)	⬆️

... and 17 files with indirect coverage changes

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