Update config to add singleton instance and add inference unit tests

[ptelang]

No description provided.

[jhrozek]

Looking good, I just left 2 questions inline, mostly for my education.

[aponcedeleonch]

Not enough to stop the PR but we need to use the provider name in the URL:

@self.router.post(f"/{self.provider_route_name}/completion")

[ptelang]

Not enough to stop the PR but we need to use the provider name in the URL:

@self.router.post(f"/{self.provider_route_name}/completion")

The provider name is added back in the URL.