Avoid using/changing `http.DefaultClient`

[maboehm]

When the user of the library does not provide a custom cient,
http.DefaultClient is used, and a custom RoundTripper is set in order to
implement authentication / retries, e.g.

stackit-sdk-go/services/serviceaccount/client.go

Lines 74 to 83 in 868d87e

	if cfg.HTTPClient == nil {
	cfg.HTTPClient = http.DefaultClient
	}
	authRoundTripper, err := auth.SetupAuth(cfg)
	if err != nil {
	return nil, fmt.Errorf("setting up authentication: %w", err)
	}
	cfg.HTTPClient.Transport = authRoundTripper

This can have unintended consequences and should be avoided in my opinion. Now,
every time I (or another package) uses the default client, it behaves
differently than expected.

I'd suggest to instead create a new client: cfg.HTTPClient = &http.Client{}.
This change would need to be done for every service.

[joaopalet]

Hi,

Thank you for opening the issue! We have created a bug ticket for this and will work on it soon

[hcsa73]

Fixed in the most recent release