schedule: fix scheduler removes region leader

[disksing]

What problem does this PR solve?

Normally, PD avoids removing leader directly when scheduling. When the leader needs to be removed, the PD inserts a transfer leader step. However, when the number of replica is set to 1, this scheme does not attempt to transfer the leader to the peer that will be created, resulting in the direct remove of the leader.

What is changed and how it works?

Constructs a list based on the current peers and the peer to be added, then looks for the target of the transferLeader in the list. If target is not found, it returns an error to cancel the schedule process.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release notes

[codecov-io]

Codecov Report

Merging #1462 into master will decrease coverage by 0.4%.
The diff coverage is 39.02%.

@@            Coverage Diff             @@
##           master    #1462      +/-   ##
==========================================
- Coverage    67.8%   67.39%   -0.41%     
==========================================
  Files         158      158              
  Lines       15262    15322      +60     
==========================================
- Hits        10348    10327      -21     
- Misses       4003     4060      +57     
- Partials      911      935      +24

Impacted Files	Coverage Δ
server/schedulers/hot_region.go	77.44% <20%> (-2.02%)	⬇️
server/schedulers/shuffle_region.go	71.92% <25%> (-4%)	⬇️
server/schedulers/adjacent_region.go	73.29% <25%> (-1.28%)	⬇️
server/schedule/region_scatterer.go	84.09% <33.33%> (-1.96%)	⬇️
server/handler.go	54.56% <33.33%> (-0.58%)	⬇️
server/schedule/replica_checker.go	80.6% <33.33%> (-7.76%)	⬇️
server/schedule/namespace_checker.go	75% <40%> (-3.34%)	⬇️
server/schedulers/balance_region.go	81.3% <40%> (-2.19%)	⬇️
server/schedule/operator.go	86.91% <61.9%> (-2.22%)	⬇️
pkg/tempurl/tempurl.go	57.14% <0%> (-14.29%)	⬇️
... and 9 more

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[nolouch]

LGTM

[nolouch]

/rebuild