impersonator: add support for service account token authentication

This change updates the impersonator logic to pass through requests
that authenticated via a bearer token that asserts a UID.  This
allows us to support service account tokens (as well as any other
form of token based authentication).

Signed-off-by: Monis Khan <mok@vmware.com>

internal/valuelesscontext/valuelesscontext.go

@@ -0,0 +1,14 @@
+// Copyright 2021 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package valuelesscontext
+
+import "context"
+
+func New(ctx context.Context) context.Context {
+	return valuelessContext{Context: ctx}
+}
+
+type valuelessContext struct{ context.Context }
+
+func (valuelessContext) Value(interface{}) interface{} { return nil }