offer getResolve to externals

[sokra]

improve types of externals configuration option
support promises in externals

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?

• when passing an function to externals
  • It may return a Promise now
  • There is a getResolve function in the first argument which is equal to the getResolve in loader context

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.