[core][C++ worker]Support specifying namespace when creating actors.

[WangTaoTheTonic]

Description

namespace implementation(doc) has been implemented in Java/Python, but still not in C++ worker.
Now user can't specify namespace when creating c++ actors, but the actors are all assigned a default empty namespace(like here).
We will support it ASAP in order to

1. make namespace working in c++ worker
2. make namespace behavior consistent in x-lang scenario.

File this issue to track it.

cc @jovany-wang

Use case

#25305 (comment)

[jovany-wang]

Thanks a lot.
FYI, the code path in Java is https://github.com/ray-project/ray/blob/master/java/runtime/src/main/java/io/ray/runtime/config/RayConfig.java#L138-L139

[stephanie-wang]

Please update the issue description and title to provide more context (explain what a namespace is and why we need it for C++ worker).

[WangTaoTheTonic]

Please update the issue description and title to provide more context (explain what a namespace is and why we need it for C++ worker).

Thanks for the reminding. I've update the description.

[WangTaoTheTonic]

PR merged. Closing this.