Support for Icechunk datasets on gcs

[Markusli]

Technical Context

• Related Issues: None
• Breaking Changes: python 3.11 -> python 3.12 and icechunk 1 -> 2

Description

Adds compatibility with GCS backed Icechunk repositories. Plugs into existing code so that if storage type is gcs, Exception is not raised and opened dataset in is returned. Also adds support for optional keywords that specify the branch, tag or snapshot a user wants to open.
To fit the pattern of existing tests I hosted a public empty dataset that is accessed when the test suite is run.

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Checklist

• Linting: Code is formatted and linted
• Tests: Tests pass. I have included new tests for these changes where applicable.
• Edge Cases: I have manually verified "unhappy paths" and edge cases beyond the basic success criteria (e.g., database connection timeouts, malformed input, strict mapping rejections).
• Documentation: I have updated README.md to reflect any new environment variables, configuration changes, or breaking updates.
• Accountability: I can explain the implementation logic for every line of code submitted.

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AI tool usage

• AI (Copilot or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated.

AI was used to help with test data preparation.

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Policy: We require a "human-in-the-loop." You are the author and are fully accountable for all submitted code. Please ensure all tool-generated content is thoroughly reviewed before submission to ensure it is not an "extractive contribution" that squanders maintainer time.