feat: also retry 5xx errors

[genisd]

retry 5xx errors. these are rare but they can occasionally happen

somewhat related to #37 and StatusCakeDev/terraform-provider-statuscake#195 even though it's not the main cause. it's a contributing cause for these

[genisd]

@tomasbasham could you take a look at this please?
I would be nice to get it in here and eventually into the terraform module as well.

[tomasbasham]

@genisd Thanks for your contribution. This repository however is the result of code generation so cannot be updated in this way. We can take these changes and add them to our templating for testing.

[genisd]

Ah that's a bummer :-)
If you could somehow get this in here that would be awesome 🥳 (we're currently running a fork which naturally is not what we want to do).

And many thanks for fixing the empty 200 body issue, one of you fixed it ❤️ , which his highly appreciated.
That was the main contributor to the two issues I linked (from my triaging research).