Add support for pip-install input

[gowridurgad]

Description:
Support for the pip-install input to install dependencies as part of the Python setup step.

Related issue:
#77

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

This PR adds support for a new pip-install input parameter that allows users to install Python dependencies directly as part of the Python setup step, eliminating the need for a separate pip install step in workflows.

• Implements installPipPackages() function to handle pip package installation
• Adds comprehensive documentation for the new feature
• Includes end-to-end tests for the pip-install functionality

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/setup-python.ts	Implements the core pip-install functionality with error handling
action.yml	Adds the pip-install input parameter definition
docs/advanced-usage.md	Documents usage examples and best practices for pip-install
README.md	Updates table of contents to include pip-install documentation
.github/workflows/e2e-cache.yml	Adds end-to-end tests for pip-install with caching scenarios
.github/workflows/e2e-cache-freethreaded.yml	Adds end-to-end tests for pip-install with free-threaded Python

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