Keep context keys private

Instead, offer type safe accessors, as documented here:

https://pkg.go.dev/context#Context

auth/interface.go

@@ -1,14 +1,13 @@
 package auth
 
 import (
+	"context"
 	"net/http"
 )
 
-var AuthCtxKey = &contextKey{"auth"}
+type contextKey string
 
-type contextKey struct {
-	name string
-}
+var authCtxKey contextKey = "auth"
 
 type AuthContext struct {
 	AuthMethod string
@@ -16,6 +15,15 @@ type AuthContext struct {
 	// TODO more?
 }
 
+func NewContext(ctx context.Context, a *AuthContext) context.Context {
+	return context.WithValue(ctx, authCtxKey, a)
+}
+
+func FromContext(ctx context.Context) (*AuthContext, bool) {
+	a, ok := ctx.Value(authCtxKey).(*AuthContext)
+	return a, ok
+}
+
 // Abstracts the authentication backend for the server.
 type AuthProvider interface {
 	// Returns HTTP middleware for performing authentication.