Include `Net::HTTPServiceUnavailable` for the debounced retry logic

[makmic]

Hi,

in our experience Deepl's API is not too stable and will raise Net::HTTPServiceUnavailable quite frequently occasion.
The client currently already implements a retry logic with execute_request_with_retries, but that only catches errors of these types:

• Net::HTTPBadResponse
• Net::HTTPServerError
• Net::HTTPFatalError
• Timeout::Error
• SocketError

Could you expose that array to be easily patchable and/or extend the list by Net::HTTPServiceUnavailable?

Thanks!