Initial implementation of radix tree-based cache (#678)

* Initial implementation of radix tree-based cache

* Adding copyright

* Updated tree.go. GPU -> Pod

* Migrated tree.go from gangmuk/prefix_and_load_aware_routing branch

* Changed to vllm/aibrix and Removed updatedMapping argument in AddPrefix

* Removed unused functions (Detokenizeation)

* Updated import utils to vllm/aibrix

* Updated AddPrefix to handle modeltopod and updating all related nodes together

* Updated evictNode and MatchPrefix to pass test

* Updated matchPrefixHelper to return empty list not nil

* Moved up LPRadixCache definition before function definitions

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Co-authored-by: Gangmuk <[email protected]>