feat!: add noop client implementation

[giortzisg]

Description

This adds the Client interface and noop and default client implementations for the SDK. This better aligns the behavior with the current client spec and removes the need to always nil check any client.

Issues

• resolves: Finalize noop client prerequisite #1366
• resolves: GO-153

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Reminders

• Add GH Issue ID & Linear ID
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Changelog Entry

• add NoopClient implementation. The SDK now returns a non-nil noop client and handles the previous nil checks with checking for a disabled client.

[linear-code]

GO-153

[cursor]

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