fix: specify the type for handler

[jannyHou]

Fixes the following build error on my local:

packages/rest/src/router/external-express-routes.ts(53,34): error TS2345: Argument of type 'Handler' is not assignable to parameter of type 'Application'.
  Type 'Handler' is missing the following properties from type 'Application': init, defaultConfiguration, engine, set, and 61 more.
npm ERR! Darwin 17.7.0
npm ERR! argv "/Users/jannyhou/.nvm/versions/node/v10.15.3/bin/node" "/Users/jannyhou/node_modules/.bin/npm" "run" "build"
npm ERR! node v10.15.3
npm ERR! npm  v3.10.6
npm ERR! code ELIFECYCLE
npm ERR! @loopback/[email protected] build: `lb-tsc es2017 --outDir dist`
npm ERR! Exit status 2
npm ERR! 
npm ERR! Failed at the @loopback/[email protected] build script 'lb-tsc es2017 --outDir dist'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the @loopback/benchmark package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     lb-tsc es2017 --outDir dist
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs @loopback/benchmark
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls @loopback/benchmark
npm ERR! There is likely additional logging output above.
npm ERR! Please include the following file with any support request:
npm ERR!     /Users/jannyhou/2019/passport-strategy/loopback-next/benchmark/npm-debug.log
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! [email protected] build: `node bin/run-lerna run build`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the [email protected] build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/jannyhou/.strong-registry/official.cache/_logs/2019-05-27T02_24_01_852Z-debug.log

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

[raymondfeng]

This is problematic. The express.static() returns a RequestHandler, which is NOT an instance of Application. The original compilation error might be caused by something else.