util/cache: add package for general-purpose caching

This package allows caching arbitrary key/value pairs in-memory, along
with an interface implemented by the cache types.

Extracted from #7493

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ic8ca820927c456721cf324a0c8f3882a57752cc9

util/cache/none.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package cache
+
+// None provides no caching and always calls the provided FillFunc.
+//
+// It is safe for concurrent use if the underlying FillFunc is.
+type None[K comparable, V any] struct{}
+
+// Get always calls the provided FillFunc and returns what it does.
+func (c None[K, V]) Get(_ K, f FillFunc[V]) (V, error) {
+	v, _, e := f()
+	return v, e
+}
+
+// Forget implements Cache.
+func (c None[K, V]) Forget() {}