Allow chromeless applications to render via non-/app routes

[eliperelman]

Summary

This PR allows applications to be rendered independently of using /app routes. Updates the frontend application service to make apps routable via appRoute during registration.

• In order to utilize a custom appRoute in the routing components, we need to pass more than just a mount function. Going down this route made me realize that if we re-structured the order of when we load the necessary app service bits, and doing a state-lift with much of the data, we could co-locate all the business logic in the app service, make the implementation simpler, and also easier to test while expanding the test coverage.
• The observables for holding the apps and legacy apps were only being used inside the app service class, and we weren't using any reactive functionality, only getValues, so I simplified to just unwrap them to their maps.
• During my work on this, changes were landed that converted the app container component to FunctionComponent, so I took the opportunity to update the relevant components to follow suit.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

Dev Docs

Allow applications to routable from paths that do not start with /app. This is first enabled via the appRoute flag during UI application registration:

export class MyPlugin implements Plugin {
  setup({ application }) {
    application.register({
      id: 'my-app',
      // App can be accessed via the /my-app route
      appRoute: '/my-app',
      async mount(context, params) {
        const { renderApp } = await import('./application');
        return renderApp(context, params);
      },
    });
  }
}

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: b70535f

[pgayvallet]

retest

[mshustov]

LGTM. I haven't tested manually tho.
Could you create a separate issue to add the full set of functional tests after #52161 ?

[eliperelman]

retest

[legrego]

Removing review request for @elastic/kibana-security since it doesn't look like this touches anything we own. Let me know if you'd still like a review!

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 0bd62f2

History

• 💚 Build #15778 succeeded 595b561
• 💚 Build #15556 succeeded c0eada9
• 💔 Build #15520 failed 9d0199d
• 💚 Build #15436 succeeded 5fc04ce
• 💔 Build #15362 failed 0134993

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