Add option to have structured JSON logging for _all_ API server output

[ashb]

Previously gunicorn/uvicorn each installed their own log handlers and
formatters, bypassing Airflow's structlog ProcessorFormatter entirely. This
meant:

• Gunicorn set up its own StreamHandler on gunicorn.error / gunicorn.access,
  so those records never went through structlog.
• Uvicorn workers called logging.config.dictConfig(LOGGING_CONFIG) on startup,
  overwriting the structlog configuration that was applied before gunicorn
  started.
• HTTP access log lines came from uvicorn's built-in access logger
  (unstructured).
• Python warnings.warn() calls and unhandled exceptions bypassed structlog
  too.
• logs emitted via structlog went to stdout; logs emitted via stdlib ended up on
  stderr

Now:

• AirflowGunicornLogger overrides setup() to skip installing gunicorn's own
  handlers and lets records propagate to root (where structlog is configured).
• AirflowUvicornWorker sets log_config=None so uvicorn doesn't clobber the
  logging config, and access_log=False since we handle that ourselves.
• HttpAccessLogMiddleware replaces uvicorn's access log: one structured event
  per request with method, path, status code, duration (µs), client address,
  and the x-request-id header bound to the structlog context for the request
  lifetime. Health-check paths are excluded to avoid noise.
• configure_logging() now explicitly silences uvicorn.access / gunicorn.access
  and routes uvicorn.error / gunicorn.error through the default handler.
• Python warnings are intercepted and emitted as structured py.warnings log
  events instead of going to stderr.
• In JSON log mode, unhandled exceptions are emitted via structlog rather than
  the plain-text default sys.excepthook.
• All logs are sent to stdout.

Pics of what it looks like

(non json version)

(json version)

[vincbeck]

Not an expert in some areas but overall looks good! Looks like a nice improvement!