feat: add leader node names to v1beta2.nodepool

[tnsimon]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Store node name with leader for yurthub.

Which issue(s) this PR fixes:

Fixes #2295

Special notes for your reviewer:

/assign @rambohe-ch

Does this PR introduce a user-facing change?

NONE

other Note

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.70%. Comparing base (0e6c0d4) to head (4589beb).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...nager/controller/hubleader/hubleader_controller.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2297   +/-   ##
=======================================
  Coverage   45.70%   45.70%           
=======================================
  Files         408      408           
  Lines       27146    27151    +5     
=======================================
+ Hits        12407    12410    +3     
- Misses      13516    13517    +1     
- Partials     1223     1224    +1     

Flag	Coverage Δ
unittests	45.70% <86.66%> (+<0.01%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rambohe-ch]

/LGTM