Update support for type hints

[ViktorSky]

This pull request introduces comprehensive type annotations to the firebase_admin codebase using PEP 484 standards. All changes have been validated with Pyright in strict mode to enforce consistency and catch potential type errors early.

Goals

• Enhance developer experience with precise autocomplete and type hints.
• Catch type-related bugs upfront via static analysis.

Tools

• Pyright in strict mode for robust type-checking.
• Manual review of ambiguous or potentially problematic sections.

Configuration Additions

• pyrightconfig.json: config file created to declare strict-mode settings.

• firebase_admin/_typing.py: A new central module that defines all custom type aliases, TypedDicts, and Protocols. By consolidating these shared types in one place, we simplify extension and reuse across the package and avoid circular-import issues.

[ViktorSky]

@jamesdaniels @lahirumaramba
Could someone please assign a reviewer to this PR? The previous one was completely ignored, and I closed it because it conflicted with the new Firebase version.

[ViktorSky]

This feature requires PR #713 to be approved before linters can take advantage of it.

[ViktorSky]

tests/test_db.py:907 creates a firebase_admin.db.Reference instance without client.

In this class, the methods assume that client will never be None.

Can you help me here?
Should I add a line that creates an instance when it's None? Or should I add validations in each part where it's referenced?