pd-api: get regions with top size

[liukun4515]

What problem does this PR solve?

• add HTTP API and pd-ctl to get the regions with the top size.

What is changed and how it works?

• add HTTP API for getting regions with the top size.
• add pd-ctl command.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Code changes

• Has HTTP API interfaces change

Side effects

no

Related changes

• Need to update the documentation

[disksing]

Don't forget to update the api.raml file too.

[rleungx]

LGTM.

[liukun4515]

PTAL @disksing