Allow to provide user config as async function

[stalniy]

Short description of the problem:

It's possible to pass only sync function for custom webpack configuration (https://github.com/ionic-team/ionic-app-scripts/blob/master/src/util/config.ts#L410)

What behavior are you expecting?

It would be useful to be able to provide async function for custom webpack configuration.

Steps to reproduce:

1. Create a custom configuration file for webpack which exports async function
2. Specify this file in package.json config.ionic_webpack
3. Run ionic serve

Ionic app scripts doesn't wait for async function to finish and it looks like it fallbacks to default webpack configuration.

Which @ionic/app-scripts version are you using?
3.1.0

Other information:

Webpack@^2.0.0 allows to provide configuration as async function: webpack/webpack#2697 (comment)

Use case:

I want to introspect my server's graphql schema and pass results into application via webpack.DefinePlugin. This is needed for Apollo to properly match types (more details - https://www.apollographql.com/docs/react/recipes/fragment-matching.html).

As a workaround it's possible to write this information to file but I don't want to manage temporary files.