Add load balancer for backend nodes

[ksysoev]

This pull request adds a load balancer for backend nodes. It includes the following changes:

• Added a LoadBalancerNode struct to represent each backend node.

• Refactored the LoadBalancer struct and related code to use LoadBalancerNode instead of BackendNode.

• Implemented the NewLoadBalancer function to create a new instance of LoadBalancer with the given backends.

• Implemented the Handle method to handle incoming requests by sending them to the least busy backend.

• Implemented the getLeastBusyNode method to return the least busy backend node.

• Added unit tests for the NewLoadBalancer, getLeastBusyNode, and Handle methods.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.51%. Comparing base (4b794d1) to head (e6d21e1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   92.21%   92.51%   +0.29%     
==========================================
  Files          21       22       +1     
  Lines         874      908      +34     
==========================================
+ Hits          806      840      +34     
  Misses         50       50              
  Partials       18       18              

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