feat(providers): Add support for fetching from k8s secrets

[bersalazar]

Allows fetching values from Kubernetes secrets.

Authentication to the Kubernetes cluster is managed by referencing the local kubeconfig.

Covers #185

[yxxhero]

@bersalazar could you add some tests for this feature?

[aslafy-z]

What about config maps? Maybe we can prep to support them in the future by leaving room in the api for them?

Maybe /[namespace/][apiVersion/kind/]name[#path.to.key]

[bersalazar]

What about config maps? Maybe we can prep to support them in the future by leaving room in the api for them?

Maybe /[namespace/][apiVersion/kind/]name[#path.to.key]

Definitely worth considering. Maybe in a future PR, happy to raise an issue for it once this one's merged.

[aslafy-z]

What about config maps? Maybe we can prep to support them in the future by leaving room in the api for them?

Maybe /[namespace/][apiVersion/kind/]name[#path.to.key]

Definitely worth considering. Maybe in a future PR, happy to raise an issue for it once this one's merged.

I still think we should ensure this implementation is future proof to reduce chances of breaking changes when we implement this.

[bersalazar]

@bersalazar could you add some tests for this feature?

@yxxhero added it

[yxxhero]

@bersalazar please see my newest comments. it will be perfect after you update.

[bersalazar]

What about config maps? Maybe we can prep to support them in the future by leaving room in the api for them?
Maybe /[namespace/][apiVersion/kind/]name[#path.to.key]

Definitely worth considering. Maybe in a future PR, happy to raise an issue for it once this one's merged.

I still think we should ensure this implementation is future proof to reduce chances of breaking changes when we implement this.

@aslafy-z sure thing, I have modified the proposed API so its more generic and can cover ConfigMaps as well, eventually. Thanks for suggesting! it makes sense to avoid future breaking changes, as you mentioned.

[bersalazar]

@bersalazar please see my newest comments. it will be perfect after you update.

@yxxhero I've had a look at them and pushed updates, including unit tests.

[aslafy-z]

Thank you for taking my point into account! Here's two little nit so we make the doc obvious for the secretref case

[yxxhero]

@bersalazar please fix lint error.

[bersalazar]

@bersalazar please fix lint error.

@yxxhero all good now, thanks.

Do you mind having a look at helmfile/helmfile#1212?
It's not directly related to this PR, but it's one of the first use cases for this new k8s provider.
Would appreciate understanding how to deal with passing the kube context down to other helmfiles, or at least see if we need to raise an issue for it.