core/internal/kubernetes: refactor PKI fully

We move ad-hoc certificate/key creation to a little declarative,
future-inspired API.

The API is split into two distinct layers:
 - an etcd-backed managed certificate storage that understands server
   certificates, client certificates and CAs
 - a Kubernetes PKI object, that understands what certificates are
   needed to bring up a cluster

This allows for deduplicated path names in etcd, some semantic
information about available certificates, and is in general groundwork
for some future improvements, like:
 - a slightly higher level etcd 'data store' api, with
   less-stringly-typed paths
 - simplification of service startup code (there's a bunch of cleanups
   that can be still done in core/internal/kubernetes wrt. to
   certificate marshaling to the filesystem, etc)

Test Plan: covered by existing tests - but this should also now be nicely testable in isolation!

X-Origin-Diff: phab/D564
GitOrigin-RevId: a58620c37ac064a15b7db106b7a5cbe9bd0b7cd0
diff --git a/core/internal/kubernetes/provisioner.go b/core/internal/kubernetes/provisioner.go
index c227fdf..c864715 100644
--- a/core/internal/kubernetes/provisioner.go
+++ b/core/internal/kubernetes/provisioner.go
@@ -24,10 +24,9 @@
 	"os"
 	"path/filepath"
 
-	"git.monogon.dev/source/nexantic.git/core/internal/common/supervisor"
-
 	"go.uber.org/zap"
 
+	"git.monogon.dev/source/nexantic.git/core/internal/common/supervisor"
 	"git.monogon.dev/source/nexantic.git/core/internal/storage"
 	"git.monogon.dev/source/nexantic.git/core/pkg/fsquota"