feat(problems): customizable errors key

Author: ConsoleTVs

README.md

@@ -12,6 +12,8 @@ for HTTP APIs (RFC 9457), making it easy to create standards-compliant APIs.
   allowing handlers to return arbitrary errors.
 - Error Handling with Responder Interface: Any returned error is checked for Responder
   implementation, providing flexibility in crafting responses.
+- Automatic error stack when a failure is provided. Errors and problems automatically craft
+  an error stack in an specific field.
 - Built-In Response Builder: Quickly build and customize responses while adhering to best
   practices like setting appropriate HTTP headers.
 - Middleware Support: Seamlessly integrate custom or provided middlewares for cleaner,

problem.go

@@ -141,6 +141,10 @@ type ProblemControls struct {
 	// of errors from the error it comes from (if any).
 	Errors ProblemControlsResolver[bool]
 
+	// ErrorsKey determines the key to use when appending the
+	// error stack when [ProblemControls]'s Errors returns true.
+	ErrorsKey ProblemControlsResolver[string]
+
 	// Response allows customizing the actual Builder response
 	// that a [Problem] should be resolved to.
 	Response ProblemControlsResolver[Builder]
@@ -181,6 +185,10 @@ func defaultedProblemControls(controls ProblemControls) ProblemControls {
 		controls.Errors = defaultProblemControlsErrors
 	}
 
+	if controls.ErrorsKey == nil {
+		controls.ErrorsKey = defaultProblemControlsErrorsKey
+	}
+
 	if controls.Response == nil {
 		controls.Response = defaultProblemControlsResponse
 	}
@@ -237,6 +245,11 @@ func defaultProblemControlsErrors(problem Problem, request *http.Request) bool {
 	return true
 }
 
+// defaultProblemControlsInstance is the default value for the [ProblemControls] instance.
+func defaultProblemControlsErrorsKey(problem Problem, request *http.Request) string {
+	return "errors"
+}
+
 // ProblemControlsResponseFrom is a helper that generates a [ProblemControlsResolver] with
 // the given response content types. It is very useful to map mimes to responses.
 func ProblemControlsResponseFrom(responses map[string]Builder) ProblemControlsResolver[Builder] {
@@ -460,7 +473,10 @@ func (problem Problem) Defaulted(request *http.Request) Problem {
 			messages[i] = trace.Error()
 		}
 
-		problem = problem.With("errors", messages)
+		problem = problem.With(
+			controls.ErrorsKey(problem, request),
+			messages,
+		)
 	}
 
 	if lower {