add support for kubernetes service discovery

This commit adds support to discover garage instances running in
kubernetes.

Once enabled by setting `kubernetes_namespace` and
`kubernetes_service_name` garage will create a Custom Resources
`garagenodes.deuxfleurs.fr` with nodes public key as the resource name.
and IP and Port information as spec in the namespace configured by
`kubernetes_namespace`.

For discovering nodes the resources are filtered with the optionally set
`kubernetes_service_name` which sets a label
`garage.deuxfleurs.fr/service` on the resources.

This allows to separate multiple garage deployments in a single
namespace.

the `kubernetes_skip_crd` variable allows to disable the creation of the
CRD by garage itself. The user must deploy this manually.

src/util/error.rs

@@ -23,6 +23,9 @@ pub enum Error {
 	#[error(display = "Invalid HTTP header value: {}", _0)]
 	HttpHeader(#[error(source)] http::header::ToStrError),
 
+	#[error(display = "kubernetes error: {}", _0)]
+	Kubernetes(#[error(source)] kube::Error),
+
 	#[error(display = "Netapp error: {}", _0)]
 	Netapp(#[error(source)] netapp::error::Error),