feat: As a user, I want to support service discovery in the stream subsystem

[spacewander]

Description

As a user, I want to support service discovery in the stream subsystem, so that we can use it to get L4 upstream dynamically.

There is already an unfinished closed PR to add this feature: https://github.com/apache/apisix/pull/4488. However, this PR is closed last year and the code has been changed a lot since that. So we can borrow some code from that PR but can't continue based on it.

[spacewander]

@zhixiongdu027 Would you like to have a try?

[zhixiongdu027]

I'm interested in doing this, but my todo list is temporarily full ~@_@~

[spacewander]

@zhixiongdu027 Thanks for your reply.

[ronething]

i will try this, please assign to me.