[Enhancement] Add namespace in java client

[drpmma]

Before Creating the Enhancement Request

• I have confirmed that this should be classified as an enhancement rather than a bug/feature.

Programming Language of the Client

Java

Summary

Add namespace in java client.

Motivation

For the purpose for the future namespace feature.

Describe the Solution You'd Like

Add namespace in ClientConfiguration.

Describe Alternatives You've Considered

No

Additional Context

No response

[lizhanhui]

What is the purpose of "For the purpose for the future namespace feature"?
Are you guys planning to add multi-tenant support to proxy/gateway?
What is the plan for other bindings out there?
@drpmma