region: Add accelerate regions schedule API (#2599)

[ti-srebot]

cherry-pick #2599 to release-4.0

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Add one new region HTTP API.

What problem does this PR solve?

close #2593

What is changed and how it works?

Add one new region related http api to accelerate regions schedule by given key range.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Support Region HTTP API to promote regions schedule by given key range

[ti-srebot]

@Yisaer please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/pd/invitations

[Yisaer]

LGTM

[ti-srebot]

@Yisaer,Thanks for your review. However, LGTM is restricted to Reviewers or higher roles.See the corresponding SIG page for more information. Related SIGs: scheduling(slack).

[Yisaer]

/run-all-tests

[codecov-commenter]

Codecov Report

Merging #2687 into release-4.0 will increase coverage by 0.15%.
The diff coverage is 46.34%.

@@               Coverage Diff               @@
##           release-4.0    #2687      +/-   ##
===============================================
+ Coverage        77.00%   77.16%   +0.15%     
===============================================
  Files              205      205              
  Lines            22276    22317      +41     
===============================================
+ Hits             17154    17220      +66     
+ Misses            3810     3790      -20     
+ Partials          1312     1307       -5     

Impacted Files	Coverage Δ
server/api/region.go	70.21% <45.00%> (-3.62%)	⬇️
server/api/router.go	99.30% <100.00%> (+<0.01%)	⬆️
server/schedule/operator/step.go	65.40% <0.00%> (-2.71%)	⬇️
server/region_syncer/client.go	79.38% <0.00%> (-2.30%)	⬇️
pkg/btree/btree.go	86.84% <0.00%> (-0.81%)	⬇️
server/member/leader.go	71.20% <0.00%> (-0.78%)	⬇️
server/schedule/operator_controller.go	81.41% <0.00%> (+0.16%)	⬆️
server/cluster/cluster.go	81.01% <0.00%> (+0.31%)	⬆️
server/handler.go	50.76% <0.00%> (+0.43%)	⬆️
server/grpc_service.go	59.84% <0.00%> (+0.77%)	⬆️
... and 6 more

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