fix LoadDefaultConfig optFns

[kentokento]

I want to use config.LoadOptionsFunc as below

    // var opts []func(*config.LoadOptions) error
    var opts []config.LoadOptionsFunc // now this will result in an error
    if c.AccessKey != "" && c.SecretKey != "" {
        opts = append(opts, config.WithCredentialsProvider(
            &credentials.StaticCredentialsProvider{
                Value: aws.Credentials{
                    AccessKeyID:     c.AccessKey,
                    SecretAccessKey: c.SecretKey,
                },
            },
        ))
    }

    if c.Profile != "" {
        opts = append(opts, config.WithSharedConfigProfile(c.Profile))
    }
    if c.Filename != "" {
        opts = append(opts, config.WithSharedConfigFiles([]string{c.Filename}))
    }
    return config.LoadDefaultConfig(context.TODO(), opts...)

[lucix-aws]

We can't do this, it's an API break. See previous discussion in #2193.