[feat][admin] Add offload managedLedgerOffloadThreshold RestAPI and CLI tools

[tjiuming]

Motivation

Add offload managedLedgerOffloadThreshold RestAPI and CLI tools

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• [ ] Dependencies (add or upgrade a dependency)
• [ ] The public API
• [ ] The schema
• [ ] The default values of configurations
• [ ] The threading model
• [ ] The binary protocol
• [x] The REST endpoints
• [x] The admin CLI options
• [ ] Anything that affects deployment

Documentation

• [ ] doc
• [ ] doc-required
• [x] doc-not-needed
• [ ] doc-complete

Matching PR in forked repository

PR in forked repository: https://github.com/tjiuming/pulsar/pull/8