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]

Caution

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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

[github-actions]

Assigning reviewers:

R: @jrmccluskey for label python.
R: @shunping for label website.

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[tvalentyn]

postcommit error is preexisting on master. spot-checked in the logs that the added postcommit test passes, and checked it manually as well.

2026-07-22T18:30:25.7229358Z [gw4] �[32mPASSED�[0m apache_beam/runners/portability/beam_plugins_it_test.py::BeamPluginsIT::test_beam_plugins_staging 

[tvalentyn]

precommit transforms: also this one: #39456