Middleware implementation prevents use of middleware that requires the `next` function

Taking over the next function limits the ability to use middleware that requires passing the next function, which is the case with some functions in bitgo's common library.

api-ts/packages/express-wrapper/src/middleware.ts

Lines 6 to 9 in 039e72e

    
           export type MiddlewareFn<T extends {}> = ( 
        
             req: express.Request, 
        
             res: express.Response, 
        
           ) => Promise<T>;

	export type MiddlewareFn<T extends {}> = (
	req: express.Request,
	res: express.Response,
	) => Promise<T>;