failover command previously gave EOF

[bseto]

./_build/kvctl failover shard 1 -n test-ns -c test-cluster0 --preferred amzFeDtPXNLPwWHIeG7Ip50cDUJT0bH1O0uYwjm7
error: EOF

after fix

./_build/kvctl failover shard 1 -n test-ns -c test-cluster0 --preferred amzFeDtPXNLPwWHIeG7Ip50cDUJT0bH1O0uYwjm7
failover shard 1 successfully, new master id: amzFeDtPXNLPwWHIeG7Ip50cDUJT0bH1O0uYwjm7.

ShouldBindJSON would give an error EOF because preferred_node_id was not in the body
https://github.com/apache/kvrocks-controller/blob/unstable/server/api/shard.go#L125

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.13%. Comparing base (6c56470) to head (83baac3).
⚠️ Report is 83 commits behind head on unstable.

Files with missing lines	Patch %	Lines
cmd/client/command/failover.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #344      +/-   ##
============================================
+ Coverage     43.38%   47.13%   +3.74%     
============================================
  Files            37       45       +8     
  Lines          2971     4428    +1457     
============================================
+ Hits           1289     2087     +798     
- Misses         1544     2132     +588     
- Partials        138      209      +71     

Flag	Coverage Δ
unittests	47.13% <0.00%> (+3.74%)	⬆️

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