Make slog Handler backwards compatible when logging custom errors

[ten4o]

Current behavior

When using slog Handler to log custom errors, the error is not converted to string and is not logged correctly.
Simple example to reproduce the problem:

package main

import (
	"fmt"
	"log/slog"
	"os"

	"code.cloudfoundry.org/lager/v3"
)

type CustomError struct {
	error
	statusCode int
}

func NewCustomError(err error, statusCode int) error {
	return &CustomError{
		error:      err,
		statusCode: statusCode,
	}
}

func main() {
	lagerLog := lager.NewLogger("component")
	lagerLog.RegisterSink(lager.NewWriterSink(os.Stdout, lager.DEBUG))
	err := NewCustomError(fmt.Errorf("error message"), 401)

	lagerLog.Error("this works", err)

	slogLog := slog.New(lager.NewHandler(lagerLog))
	slogLog.Error("this does not work", "error", err)
}

This behaviour is not backwards compatible as for parameters of type error error.Error() is called in lager.Error() but not in the slog Handler.

Desired behavior

The desired behaviour is to call Error() for parameters of type error in the slog Handler.

Affected Version

v3.0.3