Add support for middleware property to govuk-prototype-kit-config.json file

[lfdebrux]

What

Allow plugin authors to add middleware property to govuk-prototype-kit-config.json file. If they provide the path to a valid module in their package, then we will require that module and call the exported function to obtain a middleware function, which we will then attach to the prototype app.

Why

See #1798 for details.

Who needs to work on this

Developer

Who needs to review this

Developer, Tech lead

Done when

• Plugin authors can add middleware to plugins, users can add middleware when they install plugins

[lfdebrux]

In govuk-prototype-kit-config.json, type of middleware should be string | [string]. If any of the strings are not a valid path to a module within the plugin package, that is an error.

Middleware module should export one function that returns a middleware function: module.exports: () => (req, res, next) => void.

In future the kit will supply arguments to module.exports to allow configuration of the middleware, however that is out of scope for this ticket.

[joelanman]

closed until we investigate as per #1829