Add type annotations

[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]

Before approving this request, I must warn you that I cannot use pylint, because the configured version of astroid can only be used on versions lower than py3.11.
I also left some comments in the code about potential runtime issues.

• firebase_admin._messaging_encoder._Validators
  Why not @staticmethod? Perhaps there were past issues or subclassing considerations?

• firebase_admin.remote_config._ConditionEvaluator.evaluate_conditions
  You probably need to change condition.get('name') to condition['name']

• firebase_admin.remote_config._ConditionEvaluator._compare_semantic_versions
  bad docstring

• firebase_admin.remote_config._ConditionEvaluator.evaluate
  parameter_value_wrapper.get('value') is only valid if None is a valid argument to _Value

• firebase_admin.functions.TaskQueue._update_task_payload
service_account_email is accessed on a type that might not have that attribute

[ViktorSky]

To upload to pypi you must approve #713, because linters like mypy will ignore it since it is not marked as safe