[Feature] The new implement of Selector in Nacos 2.0.

[brotherlu-xcq]

Is your feature request related to a problem? Please describe.

The old implement of Selector in Nacos 1.x just a simple and inflexible implement, cannot extend by user self. when we have another scene, we must integrate the new function to Nacos, it's not a good idea. you can find in the issue #5612 .

So in Nacos 2.0, we want to introduce an new Implement of Selector to make the Selector more flexible and friendly for users.

Describe the solution you'd like

We will make the Selector as two part for user which can be implement by their self if they need. one is still Selector which only provide select instances logic, another is SelectContextBuilder which provide the context used by Selector. the main work flow of new implement like below.

you can find more detail description in detail design.

We split the whole feature as parts like below:

part	description	sub issue	assign	status
1	Define the workflow, include Selector, SelectorManager, SelectorContextBuilder and add unit test for the workflow	#6356	@brotherlu-xcq	done
2	Implement the built-in LableSelecotr, CMDBSelectoerContextBuilder, which can match the most scene and the implement in Nacos 1.x, Add unit test for the built-in implement	#6365	@brotherlu-xcq	done
3	Integrate this workflow to Nacos 2.0, including all sence like Grpc, Http and add unit test.	#6430	@brotherlu-xcq	done
4	add the use example		@brotherlu-xcq
5	add the use document		@brotherlu-xcq

Describe alternatives you've considered
No

Additional context
No