Make gateway active

[Egor-S]

This PR changes the communication scheme between the server, job instance, and gateway.

Closes #799

• The gateway now runs the FastAPI app as a systemd service
• The gateway now establishes an SSH tunnel to the job instance (contrary to the previous behavior)
• The server registers and unregisters a service via secured HTTP requests to the gateway
• The gateway features an OpenAI interface
  • OpenAI requests are converted to TGI format and sent to the job instance
  • OpenAI interface supports /models query (scoped by the project)
• If run from source, the server picks the latest build version related to the HEAD

Steps to use an OpenAI interface:

1. Create a new default gateway
2. Redirect the DNS record to the gateway
3. Add the following to the service configuration (TGI)

   model:
     type: chat
     name: TheBloke/Mistral-7B-OpenOrca-AWQ
     format: tgi
     # optionally override `chat_template:` and `eos_token:` here
   

4. Run the service using dstack run
5. Make a request to gateway.<your domain> using openai python package

TODO before merge:

• Update gateway app on server start up
• Build gateway app in release workflow