feat: Enhance production readiness and scalability

[cning112]

This commit introduces a comprehensive set of improvements to make the FastFlight project more robust, secure, configurable, observable, and scalable for production environments.

Key enhancements include:

1. Configuration:

   • Implemented a centralized configuration system using Pydantic
     Settings (environment variables, .env file support) for
     Flight server, FastAPI app, and Bouncer defaults.

2. Logging:

   • Enhanced structured logging (JSON/plain) using structlog,
     configurable via settings. Improved log context and consistency.

3. Security:

   • Added token-based authentication to the Arrow Flight server.
   • Implemented API key authentication for FastAPI endpoints.
   • Integrated TLS configuration options for both Flight (gRPC)
     and FastAPI (HTTPS) services.

4. Monitoring & Metrics:

   • Integrated Prometheus monitoring via prometheus_client and
     starlette-prometheus.
   • Exposed metrics for Flight server, FastAPI, FastFlightBouncer
     (including connection pool and circuit breaker stats) via
     a /metrics endpoint.

5. Dockerfile & Deployment:

   • Overhauled Dockerfile: uses multi-stage builds, slim base
     image, and runs as a non-root user.
   • Added a health check script and Docker HEALTHCHECK instruction
     for combined Flight and FastAPI service status.
   • Default Docker CMD now runs fastflight start-all.

6. Scalability & Robustness:

   • Improved graceful shutdown handling, especially for the
     start-all CLI command managing multiple processes.
   • Verified lifecycle management of AsyncToSyncConverter.
   • Provided guidance on scaling the Flight server using a
     multi-process architecture with a load balancer.

7. Testing:

   • Significantly increased test coverage, adding tests for new
     configuration system, authentication mechanisms (Flight & FastAPI),
     metrics endpoint, health checks, and an end-to-end authenticated
     integration scenario.

8. Documentation:

   • Generated extensive documentation content covering all new
     features: configuration, security setup, TLS, monitoring,
     deployment (Docker, health checks), and scaling strategies.
     (Content provided for manual integration into markdown files).