m/n/time: add time service

This adds a bare-minimum time service based on chrony/NTP for keeping
the system clock and RTC on Metropolis nodes accurate.

It also introduces a UID/GID registry in the Metropolis node code
as this is the first unprivileged service to run on the node itself.

It does not yet use a secure time source, this is tracked as #73.

Change-Id: I873971e6d3825709bc8c696e227bece4cfbda93a
Reviewed-on: https://review.monogon.dev/c/monogon/+/319
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/node/core/time/BUILD.bazel b/metropolis/node/core/time/BUILD.bazel
new file mode 100644
index 0000000..05a938a
--- /dev/null
+++ b/metropolis/node/core/time/BUILD.bazel
@@ -0,0 +1,13 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["time.go"],
+    importpath = "source.monogon.dev/metropolis/node/core/time",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//metropolis/node:go_default_library",
+        "//metropolis/pkg/fileargs:go_default_library",
+        "//metropolis/pkg/supervisor:go_default_library",
+    ],
+)