[Meta] Plugins in the new platform

[kimjoar]

Top-level issue for plugins in the new platform. Each of the tasks below should be pulled into separate issues instead of discussing them in detail here — this is just a meta issue to collect high-level thoughts.

Tasks

• Explore "fully independent" core plugins: Explore having "independent" core plugins in the new platform #12467
• Don't start disabled plugins (d2a4257)
• Setup for typing plugins (Typed plugins in new platform #12857)
• Read plugins that don't live in the Kibana core_plugins folder, e.g. xpack (Read plugins that don't live in the Kibana core_plugins folder #14394)
• Plugins must be able to specify config (Plugins must be able to specify config #14414)

[azasypkin]

Just a quick note, I've checked how/if Angular 2+ exposes Observables in their public API:

• They use Observable as a part of their core public API signature, e.g. take a look at https://angular.io/api/common/http/HttpClient;
• RxJS is declared as a peer dependency for Angular npm package.

Angular team did a good job in explaining what peer dependencies are required and why: https://angular.io/guide/npm-packages#polyfill-packages and https://angular.io/guide/npm-packages#why-peer-dependencies. So we should do that too in our plugins docs.

Also we may want to have a narrow version range for these peer dependencies to have less troubles.

[kimjoar]

All the initial tasks are done, so I'm closing this 🎉