[wip] add raw data path to apps

[cosmicBboy]

This pull request introduces support for configuring and accessing the raw data path in Flyte apps, enabling applications to read the raw data path from the app context. The changes allow the raw data path to be set via command-line options, environment variables, or programmatically, and ensure it is correctly propagated in local and remote serving scenarios. An example FastAPI app is also added to demonstrate this new feature.

Raw Data Path Support

• Added a new raw_data_path attribute to the AppContext dataclass and exposed it via the ctx() function, allowing apps to access the raw data path. Also implemented the set_raw_data_path() function to set this value and propagate it via environment variables for thread safety. (src/flyte/app/_context.py)
• Updated the serving context (_Serve) and its factory function (with_servecontext) to accept and propagate the raw_data_path parameter, defaulting to /tmp/flyte/raw_data for local mode. The serving logic ensures the raw data path is set in the correct thread context for request handlers. (src/flyte/_serve.py)* Extended the CLI (serve.py) to accept a --raw-data-path option and set the raw data path in the app context when provided. (src/flyte/_bin/serve.py)
• Ensured that the container command for remote serving includes a template variable for the raw data path, allowing the backend to substitute the correct value at runtime. (src/flyte/app/_app_environment.py)

Documentation and Example

• Added an example FastAPI app (app_with_raw_data_path.py) that demonstrates accessing the raw data path from the app context and returning it in an API response. (examples/apps/app_with_raw_data_path.py)

[pingsutw]

Instead of updating the env var in set_raw_data_path, would it be better to just pass raw_data_path to _serve()?