fix: allow multi-part dataset IDs to support BigLake tables

[tswast]

Relaxes DatasetReference.from_string and TableReference.from_string validation.

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Fixes b/512823729 🦕

[gemini-code-assist]

Code Review

This pull request introduces a new internal module _string_references.py to centralize the parsing of dataset and table ID strings into REST resource representations, refactoring DatasetReference.from_string and TableReference.from_string to use these shared utilities. Feedback focuses on ensuring Python version compatibility for type unions, correcting PEP 8 indentation violations, programmatically sorting returned dictionaries per repository rules, and refining the logic for multi-part dataset IDs.