schedulers: add role config to shuffleRegionScheduler (#2219)

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What problem does this PR solve?

cherry-pick #2219 to fix #2177

What is changed and how it works?

Add role configuration to shuffle-region-scheduler.

Use it to balance learners:

1. add shuffle-region-scheduler

pd-ctl scheduler add shuffle-region-scheduler

or

curl -XPOST -d '{"name": "shuffle-region-scheduler"}' http://pd:2379/pd/api/v1/schedulers

2. config roles

pd-ctl scheduler config shuffle-region-scheduler set-roles learner

or

curl -XPOST -d '["learner"]'  http://pd:2379/pd/api/v1/scheduler-config/shuffle-region-scheduler/roles

Check List

Tests

• Unit test
• Manual test

Code changes

• Has HTTP API interfaces change (Don't forget to update API document)

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

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/run-all-test

[disksing]

/run-all-tests