Provide compatible types for v5.x

[legobeat]

Upgrading from 5.0.0 to 5.1.1 causes Typescript errors: https://github.com/MetaMask/json-rpc-engine/actions/runs/5660095486/job/15335070066?pr=147

MetaMask/json-rpc-engine#147 (review)

Would be great to fix the types so a new ^5.0.0 install works with code implemented towards 5.0.0.

[legobeat]

@Mrtenz Did you have a chance to look at this?

[Mrtenz]

@Mrtenz Did you have a chance to look at this?

I don't really have the time to look into this at the moment. If you want to take a look, feel free!