Remove LiteLLM and build native implementation

[lukehinds]

While LiteLLM is very useful (huge coverage of providers), we always invariably hit issues around certain idiomatic approaches taken by LiteLLM, and feel we need to take a different approach. It therefore makes sense to build our own native implementation

If possible, we should initially seek to support both LiteLLM and the CodeGate implementation (if viable and not to heavy a lift).