Allow appOptions to resolve asynchronously

[Sergej-Popov]

What: This PR adds the ability to resolve app options asynchronously.
Why: In some scenarios (e.g. multi-tenant applications) resolution of app options such as a router, i18n etc, requires loading configuration and resources asynchronously.

At the moment there are no asynchronous hooks except loadRootComponent which is insufficient.

Example:

const vueLifecycles = singleSpaVue({
  Vue,
  async appOptions () {
    const config = await loadConfiguration()

    const i18n = await  i18nFactory(config);
    const router = await  routerFactory(config);

    return {
      render: h => h(App),
      router,
      i18n
    }
  }
});

export const bootstrap = vueLifecycles.bootstrap;
export const mount = vueLifecycles.mount;
export const unmount = vueLifecycles.unmount;

[joeldenning]

I like the change, thanks for adding tests for the code and types.

Could you help with updating the documentation for single-spa-vue? Here's the file to modify:

https://github.com/single-spa/single-spa.js.org/blob/master/website/versioned_docs/version-5.x/ecosystem-vue.md

I'll merge and publish this now

[joeldenning]

I've created single-spa/single-spa.js.org#403 to track documenting this feature.

[joeldenning]

This is released in https://github.com/single-spa/single-spa-vue/releases/tag/v2.2.0

[Sergej-Popov]

Thanks for releasing so quickly. Docs: single-spa/single-spa.js.org#405