Request to support amazonka 2.0

[jhrcek]

Hello @koterpillar, since amazonka 2.0 release candidate is out (https://github.com/brendanhay/amazonka/discussions/716) I'd like to upgrade to it, but this will require this library to migrate to it too. Are you planning to upgrade this library to support it?

~If you're too busy, I could try opening a PR (not sure yet how much effort it would be).~ I opened an initial PR to help you preview the necessary changes.

Would you prefer to support both <2.0 and >=2.0 version of amazonka or would you be ok with moving to >=2.0 only? The former would likely require lots of CPP..