Async Task Interface for long running functions

[Tinyblargon]

Adds a Task interface that can be used for asynchronously awaiting the completion of the task.

The async part is enabled with the Features.AsyncTask flag in the Client struct.

Resolves #388

Supersedes:
Closes #397
Closes #389

[Copilot]

Copilot reviewed 21 out of 21 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

proxmox/vmref.go:18

• Consider using fmt.Errorf to wrap errors for a more consistent error formatting style when passing context with the error message.

return nil, nil, errors.New(VmRef_Error_Nil)

proxmox/config_qemu_disk.go:913

• Explicitly return nil for the exitStatus when an error occurs (e.g., 'return nil, err') to avoid relying on implicit zero values from named return parameters.

if err != nil { return }

proxmox/client.go:41

• [nitpick] Ensure that the Features field is actively utilized to conditionally handle async task flows or consider adding clarifying comments since its purpose is not immediately evident in the current implementation.

Features           *FeatureFlags

[Tinyblargon]

@fireflg would this new Task interface work for your use-case? Assuming the feature is enable and the real one is used instead of the dummy.