Implement global `shade` module configuration

[codebestia]

Description:

The SDK should support module-level configuration (e.g. shade.api_key = "sk_live_...") so developers can configure it once at app startup without instantiating a client object on every call. This mirrors the ergonomics of the Stripe Python SDK. The global config must be thread-safe and readable by all resource classes.

Proposed Steps:

• Create src/shade/config.py with module-level variables: api_key, api_base, environment, timeout, max_retries.
• Expose these on the top-level shade namespace via __init__.py.
• Write a get_config() helper that merges instance-level overrides with global defaults, used internally by all resource methods.
• Guard against None api_key at request time and raise AuthenticationError.

Acceptance Criteria:

• shade.api_key = "sk_live_xxx" sets the global key accessible across all resource calls.
• shade.environment = "sandbox" switches the active environment.
• Setting shade.api_key = None and then calling any resource raises AuthenticationError with a clear message.
• Global config does not bleed between threads when changed concurrently.

[Whiznificent]

gm again maintainer please assign me this issue to work on

[Elizabeth-dev270]

I enjoy solving software problems and have experience in backend development, APIs, OOP, and scalable application design.

[Oluwasuyi-Timilehin]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!