[improve][admin] Add namespace migration state query in rest API

[oneby-wang]

Motivation

The namespace migration API only supports updating the migrated flag, but there is no dedicated API to query the namespace migration state directly.

Modifications

Add a namespace migration state query API in broker and admin client, and update the related tests to use the new API.

Verifying this change

• Make sure that the change passes the CI checks.

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
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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