pdctl: add placement rules commands

[disksing]

What problem does this PR solve?

cherry-pick #2293

What is changed and how it works?

add commands:

• pd-ctl config placement-rules enable, same as config set enable placement-rules true
• pd-ctl config placement-rules disable, same as config set enable placement-rules false
• pd-ctl config placement-rules show, show all rules
• pd-ctl config placement-rules show --group=g1, show rules in a group
• pd-ctl config placement-rules show --group=g1 --id=id2, show a rule
• pd-ctl config placement-rules show --region=1, show rules match a region
• pd-ctl config placement-rules load -out=rules.json, load all rules to a file
• pd-ctl config placement-rules save -in=rules.json, save rules in a file to PD

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Check List

Tests

• Unit test

Related changes

• PR to update pingcap/docs/pingcap/docs-cn: TBD

[lhy1024]

/run-all-tests

[disksing]

/run-all-tests

[nolouch]

/merge

[sre-bot]

/run-all-tests