✨ Add multi provider

[ntnn]

Add the multi provider, allowing to use multiple providers with a single manager without conflicting cluster names.

Pseudo to instantiate:

multiProvider := multi.New(multi.Options{})
mgr := mctrl.NewManager(ctx, multiProvider, mctrl.Options{})
multiProvider.SetManager(mgr) // needed for multi to wrap the mgr when calling startFunc

provider1 := provider1.New(provider1.Options{})
// provider1.Run only expects context and mgr and can be passed as startFunc
if err := multiProvider.AddProvider(ctx, "provider1", provider1.Run}; err != nil {
    return err
}

provider2 := provider2.New(provider2.Options{})
if err := provider2.Setup(...); err != nil {
    return err
}
if err := multiProvider.AddProvider(ctx, "provider2", func(ctx context.Context, mgr mctrl.Manager) error {    
   workers := 2 // or some other options that the provider expects to be passed to .Run
    return provider2.Run(ctx, workers, mgr)
}); err != nil {
    return err
}

mgr.Start(ctx)

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[ntnn]

Will need squashing but I'll do that after reviews

[embik]

Two small nits.

[corentone]

I haven't followed all discussions in the sig sorry, but would it be possible to modify the manager to allow mulitple providers in the first place?

[embik]

I haven't followed all discussions in the sig sorry, but would it be possible to modify the manager to allow mulitple providers in the first place?

I think so, this here would give us a way to experiment and iterate on the design without breaking anything for users of multicluster-runtime not interested in running multiple providers. So I think it might be the more elegant design, helping us explore the versatility of the provider interface further.