m/n/core: factor out gRPC/TLS into rpc and identity libraries

This is an annoying large change, which started its life as me pulling
the 'let's add tests for authentication' thread, and ended up in
unifying a whole bunch of dispersed logic under two new libraries.

Notable changes:

 - m/n/core/identity now contains the NodeCertificate (now called Node)
   and NodeCredentials types. These used to exist in the cluster code,
   but were factored out to prevent loops between the curator, the
   cluster enrolment logic, and other code. They can now be shared by
   nearly all of the node code, removing the need for some conversions
   between subsystems/packages.
 - Alongside Node{,Credentials} types, the identity package contains
   code that creates x509 certificate templates and verifies x509
   certificates, and has functions specific to nodes and users - not
   clients and servers. This allows moving most of the rest of
   certificate checking code into a single set of functions, and allows
   us to test this logic thoroughly.
 - pki.{Client,Server,CA} are not used by the node core code anymore,
   and can now be moved to kubernetes-specific code (as that was their
   original purpose and that's their only current use).
 - m/n/core/rpc has been refactored to deduplicate code between the
   local/external gRPC servers and unary/stream interceptors for these
   servers, also allowing for more thorough testing and unified
   behaviour between all.
 - A PeerInfo structure is now injected into all gRPC handlers, and is
   unified to contain information both about nodes, users, and possibly
   unauthenticated callers.
 - The AAA.Escrow implementation now makes use of PeerInfo in order to
   retrieve the client's certificate, instead of rolling its own logic.
 - The EphemeralClusterCredentials test helper has been moved to the rpc
   library, and now returns identity objects, allowing for simplified
   test code (less juggling of bare public keys and
   {x509,tls}.Certificate objects).

Change-Id: I9284966b4f18c0d7628167ca3168b4b4037808c1
Reviewed-on: https://review.monogon.dev/c/monogon/+/325
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/pkg/supervisor/supervisor_testhelpers.go b/metropolis/pkg/supervisor/supervisor_testhelpers.go
index 711ed00..b2812c2 100644
--- a/metropolis/pkg/supervisor/supervisor_testhelpers.go
+++ b/metropolis/pkg/supervisor/supervisor_testhelpers.go
@@ -49,6 +49,7 @@
 	logtree.PipeAllToStderr(t, lt)
 
 	New(ctx, func(ctx context.Context) error {
+		Logger(ctx).Infof("Starting test %s...", t.Name())
 		if err := r(ctx); err != nil && !errors.Is(err, ctx.Err()) {
 			t.Errorf("Supervised runnable in harness returned error: %v", err)
 		}