Add the directory with staged files to sys.path and document the usage of --files_to_stage and --beam_plugins

[tvalentyn]

fixes: #39431

This feature also enables Beam SDK harness initialization capability for Python (discussed on dev) by supplying the following options: --file_to_stage=/path/to/my_custom_module_to_import_on_startup.py --beam_plugins=my_custom_module_to_import_on_startup

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[gemini-code-assist]

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[tvalentyn]

/gemini review

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[tvalentyn]

assign set of reviewers

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