[feature][proxy] Add clientStats API for ProxyStats

[graysonzeng]

Master Issue: https://github.com/apache/pulsar/issues/17625

Motivation

Add a new api named clientStats to proxy stats

Modifications

see issue

Verifying this change

• [x] Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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 binary protocol
• [x] The REST endpoints
• [ ] The admin CLI options
• [ ] Anything that affects deployment

Documentation

• [x] doc-required (Your PR needs to update docs and you will update later)

• [ ] doc-not-needed (Please explain why)

• [ ] doc (Your PR contains doc changes)

• [ ] doc-complete (Docs have been already added)

[github-actions[bot]]

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