feat(storage): Introduce base abstract class for storage client

[googlyrahman]

This PR introduces an abstract class to support the upcoming async client for the Python SDK. This refactor defines the public interface and enables code sharing between sync and async client, without introducing new logic.

[gemini-code-assist]

Summary of Changes

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This pull request refactors the client initialization process by introducing an abstract base class, BaseClient, which centralizes common setup procedures for client objects. This change extracts duplicated logic related to project configuration, credential management, and API endpoint resolution from the concrete Client class. The result is a cleaner, more maintainable Client implementation that leverages inheritance for shared functionality, improving consistency and reducing code redundancy across the library.

Highlights

• New Abstract Base Class: Introduced BaseClient as a new abstract base class in google/cloud/storage/abstracts/base_client.py to house common client initialization logic.
• Client Refactoring: The existing Client class in google/cloud/storage/client.py now inherits from the new BaseClient, significantly streamlining its __init__ method by delegating shared setup responsibilities.
• Centralized Initialization Logic: Moved complex logic for project and credential handling, API endpoint determination (including emulator and universe domain overrides), and universe domain validation from Client to BaseClient.

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[gemini-code-assist]

Code Review

This pull request introduces a new abstract BaseClient class in google/cloud/storage/abstracts/base_client.py to centralize common client initialization logic, including project and credential handling, API endpoint determination based on client options and environment variables (like emulator settings and universe domain), and api_key processing. The existing Client class in google/cloud/storage/client.py is refactored to inherit from this BaseClient, significantly simplifying its __init__ method by delegating most of the setup to the superclass and removing redundant imports. Review comments suggest modifying the BaseClient constructor to avoid a mutable default argument for extra_headers and removing an informal comment in the Client class regarding connection_kw_args.