fix: correct handling of endpointSlices in Kubernetes service discovery

[ChuanFF]

Description

Currently, the Kubernetes service discovery implementation in APISIX incorrectly handles EndpointSlices, treating each individual slice as the complete set of endpoints for a service. This contradicts the official Kubernetes documentation.

According to the Kubernetes EndpointSlice documentation:

"For a given service there may be multiple EndpointSlice objects which must be joined to produce the full set of endpoints; you can find all of the slices for a given service by listing EndpointSlices in the service's namespace whose kubernetes.io/service-name label contains the service's name."

The correct behavior should be to combine all EndpointSlices sharing the same kubernetes.io/service-name label to form the complete endpoint set for a service.

Solution Approach

1. Add EndpointSlices Cache

• Implemented endpoint_slices_cache to store all EndpointSlices with the structure:
  endpoint_slices_cache["k8s_id/namespace/k8s_service_name:port_name"] = {slice1, slice2, ...}
• Any changes to EndpointSlices trigger updates to this cache
• The cache is then used to assemble the complete node list for each service, which is subsequently updated in shared memory

2. Code Refactoring

• Renamed variables to better align with Kubernetes EndpointSlices data structure terminology
• Improved code readability and maintainability

3. Some Bug Fixes

• Fixed incorrect handling of addresses field: The addresses field contains a list of IP strings, not objects with .ip attributes
• Optimized node sorting logic: Simplified the nested loop structure in endpoint_buffer processing:

  -- Before (incorrect):
  for _, ports in pairs(endpoint_buffer) do
      for _, nodes in pairs(ports) do
          core.table.sort(nodes, sort_nodes_cmp)
      end
  end
  
  -- After (correct):
  for _, nodes in pairs(endpoint_buffer) do
      core.table.sort(nodes, sort_nodes_cmp)
  end

  The endpoint_buffer uses ports as keys and node lists as values, requiring only a single iteration level.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)