Make file uploads directory configurable

[brooklynbagel]

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Is your feature request related to a problem? Please describe. Right now the files directory (.files) is hard-coded into the application and is not configurable. This can be a problem when Chainlit is being run on an environment that doesn't give the user direct control of the directory the application is installed and run in.

Describe the solution you'd like Add a configuration to config.toml to override the default files directory. I'd suggest something like features.spontaneous_file_upload.files_dir

Describe alternatives you've considered Keep the status-quo which is problematic on certain environments.

Additional context I have a PR that is ready for review, https://github.com/Chainlit/chainlit/pull/2156

[rakeshvikhar]

this is very much needed