svchost/auth: token-based HostCredentials

This is the only credentials type we support right now, which just sends
an opaque token via the "Bearer" HTTP auth scheme.

svchost/auth/token_credentials.go

@@ -0,0 +1,20 @@
+package auth
+
+import (
+	"net/http"
+)
+
+// HostCredentialsToken is a HostCredentials implementation that represents a
+// single "bearer token", to be sent to the server via an Authorization header
+// with the auth type set to "Bearer"
+type HostCredentialsToken string
+
+// PrepareRequest alters the given HTTP request by setting its Authorization
+// header to the string "Bearer " followed by the encapsulated authentication
+// token.
+func (tc HostCredentialsToken) PrepareRequest(req *http.Request) {
+	if req.Header == nil {
+		req.Header = http.Header{}
+	}
+	req.Header.Set("Authorization", "Bearer "+string(tc))
+}

svchost/auth/token_credentials_test.go

@@ -0,0 +1,18 @@
+package auth
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestHostCredentialsToken(t *testing.T) {
+	creds := HostCredentialsToken("foo-bar")
+	req := &http.Request{}
+
+	creds.PrepareRequest(req)
+
+	authStr := req.Header.Get("authorization")
+	if got, want := authStr, "Bearer foo-bar"; got != want {
+		t.Errorf("wrong Authorization header value %q; want %q", got, want)
+	}
+}