[Server] Use the integration field when starting a server

[dsikka]

Summary

• The integration value defined in the server config was being ignored and the cli argument was only being considered
• Update to use both
• Also update tests and small clean-up
• For this ticket: https://app.asana.com/0/1205229323407165/1206280777419100/f

Testing

• All server tests pass

The following config file will create an openai server:

num_workers: 1
num_streams: 1
integration: openai
endpoints:
  - task: text_generation
    model: "hf:mgoin/TinyStories-1M-ds"
    kwargs:
      {"continuous_batch_sizes": [4, 8, 16], "force_max_tokens": True}