Reorganize the `config` module

What does this merge request do and why?

This MR reorganizes the config module to make it less error-prone, stable, and generic. Note that we are not changing the configuration logic and are still using env variables with slightly updated names and the additional type validation provided by pydantic and pydantic_settings.

Changes:

• all env variables related to the old config.py have been prefixed with AIGW_, i.e., AI Gateway.
• we have 8 groups of options: logging, fastapi, auth, google_cloud_profiler, f, snowplow, vertex_text_model, model_engine_concurrency_limits.
• setting group options now requires adding an extra underscore to the env variable. E.g., AIGW_FASTAPI__API_HOST is translated to config.fast_api.api_host.
• legacy env variables, such as F_IS_THIRD_PARTY_AI_DEFAULT, have been removed.
• several env variables, such as PALM_TEXT_PROJECT have been renamed.

Related conversation - https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/359#note_1680248624

Blocked by - !530 (merged)

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure a local Docker image built successfully.

   docker buildx build --platform linux/amd64 \
     -t ai-gateway:dev .

3. Run a local service on Docker.

   docker run --platform linux/amd64 --rm \
     -p 5052:5052 \
     -e AIGW_AUTH__BYPASS_EXTERNAL=true \
     -v $PWD:/app -it ai-gateway:dev

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.