refactor: implement builder pattern for OmnectDeviceServiceClient with lifecycle management

[JanZachmann]

Summary

This PR refactors OmnectDeviceServiceClient to implement a type-safe builder pattern with lifecycle management, improving encapsulation and maintainability.

Key Changes

• Builder Pattern: Implemented as the only way to create OmnectDeviceServiceClient, with private struct fields
• Optional Certificate Setup: Certificate creation is now optional and injectable via with_certificate_setup()
• Optional Publish Endpoint: Publish endpoint registration is now optional via with_publish_endpoint()
• Code Organization: Extracted service client creation into dedicated build_service_client() function
• Simplified Certificate Module: Made CreateCertPayload public and removed intermediate WorkloadCertPayload
• Version Bump: Updated to 1.0.6
• Dockerfile Fix: Changed to copy entire src directory structure

Benefits

• Better encapsulation through private fields and builder pattern
• Clearer lifecycle management with explicit shutdown
• Looser coupling via dependency injection (closures for setup)
• More flexible configuration (both features are optional)
• Easier testing with injectable dependencies

[empwilli]

LGTM, minor nit

[empwilli]

I wonder whether we really need to make OmnectDeviceServiceClientBuilder generic over the Fut type. If not, we can drop the _phantom marker and simplify it a little bit.

If I understand the code correctly, Fut is only necessary as a bound for F for the result type? wouldn't it simplify things if we just had

pub struct OmnectDeviceServiceClientBuilder<F>
where
    F: FnOnce(CreateCertPayload) -> std::future::Future<Output = Result<()>>,
{
...

instead?

Alternatively, we should provide a default type for the Fut in the generic type declaration to simplify things at the call site.

[JanZachmann]

Good catch! I've simplified the builder by removing the Fut generic parameter and PhantomData. Changes:

• Builder is no longer generic over F and Fut
• Uses boxed trait objects for the certificate setup function
• Split the type alias into two for better readability:
  • type CertSetupFuture = Pin<Box<dyn Future<Output = Result<()>>>>;
  • type CertSetupFn = Box<dyn FnOnce(CreateCertPayload) -> CertSetupFuture>;

The API remains identical for callers, and the small runtime cost (heap allocation + dynamic dispatch) is negligible since it only happens once during initialization. The improved simplicity is worth the trade-off.