Expose `Request` Typescript interface from core types

[saintybalboa]

Is your feature request related to a problem? Please describe.

I need to assign a type to function arguments of the middleware object (MiddlewareObj). Digging through the middy codebase I found the Request interface which seems to be what I need but it isn't exported so cannot be used.

Describe the solution you'd like

Export the Request interface so that consumers can assign the type to the middleware functions.

const middlewareObj = {
    before: (request: Request) => {
      ...
    }
}

Describe alternatives you've considered

If there's already a type available for this it might be worth adding this to the docs.

Additional context

N/A

[willfarrell]

Would you be able to open a PR against the v4 branch to add this in?

[saintybalboa]

@willfarrell here's a link to the PR. Let me know if there's anything I've missed / needs updating.