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

[jonball4]

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>;