Switch to a slimmer distroless base image.

At a high level, it switches us to a distroless base container image, but that also includes several related bits:

- Add a writable /tmp but make the rest of our filesystems read-only at runtime.

- Condense our main server binaries into a single pinniped-server binary. This saves a bunch of space in
  the image due to duplicated library code. The correct behavior is dispatched based on `os.Args[0]`, and
  the `pinniped-server` binary is symlinked to `pinniped-concierge` and `pinniped-supervisor`.

- Strip debug symbols from our binaries. These aren't really useful in a distroless image anyway and all the
  normal stuff you'd expect to work, such as stack traces, still does.

- Add a separate `pinniped-concierge-kube-cert-agent` binary with "sleep" and "print" functionality instead of
  using builtin /bin/sleep and /bin/cat for the kube-cert-agent. This is split from the main server binary
  because the loading/init time of the main server binary was too large for the tiny resource footprint we
  established in our kube-cert-agent PodSpec. Using a separate binary eliminates this issue and the extra
  binary adds only around 1.5MiB of image size.

- Switch the kube-cert-agent code to use a JSON `{"tls.crt": "<b64 cert>", "tls.key": "<b64 key>"}` format.
  This is more robust to unexpected input formatting than the old code, which simply concatenated the files
  with some extra newlines and split on whitespace.

- Update integration tests that made now-invalid assumptions about the `pinniped-server` image.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

internal/concierge/server/server.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"time"
 
 	"github.com/spf13/cobra"
@@ -16,6 +17,10 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/serializer"
 	genericapiserver "k8s.io/apiserver/pkg/server"
 	genericoptions "k8s.io/apiserver/pkg/server/options"
+	"k8s.io/client-go/pkg/version"
+	"k8s.io/client-go/rest"
+	"k8s.io/component-base/logs"
+	"k8s.io/klog/v2"
 
 	"go.pinniped.dev/internal/certauthority/dynamiccertauthority"
 	"go.pinniped.dev/internal/concierge/apiserver"
@@ -231,3 +236,21 @@ func getAggregatedAPIServerConfig(
 	}
 	return apiServerConfig, nil
 }
+
+func Main() {
+	logs.InitLogs()
+	defer logs.FlushLogs()
+
+	// Dump out the time since compile (mostly useful for benchmarking our local development cycle latency).
+	var timeSinceCompile time.Duration
+	if buildDate, err := time.Parse(time.RFC3339, version.Get().BuildDate); err == nil {
+		timeSinceCompile = time.Since(buildDate).Round(time.Second)
+	}
+	klog.Infof("Running %s at %#v (%s since build)", rest.DefaultKubernetesUserAgent(), version.Get(), timeSinceCompile)
+
+	ctx := genericapiserver.SetupSignalContext()
+
+	if err := New(ctx, os.Args[1:], os.Stdout, os.Stderr).Run(); err != nil {
+		klog.Fatal(err)
+	}
+}