Pick up rates from the core listing

[adubovik]

Currently the rates (per-model prompt and completion prices) are read from MODEL_RATES env var.

We need to pick the rates instead from the listing served by the core.

Note, that this issue must be resolved first: epam/ai-dial-core#53

[nepalevov]

Please remove the "MODEL_RATES" env variable from the Dockerfile as well. It confuses when the variable is declared empty by default in docs but actually being defined on container level.