New platform initial setup

[kimjoar]

Initial architecture for #9675, including quite a bit of example code.

(The actual code diff is +3k~ lines, but tests and snapshots adds quite a few lines)

For explanation on setup see platform/README.md in the PR.

How files are required (can be a help when understanding the general flow):

Todos

(these are NOT the todos for this PR, but things I've thought about while building this)

• Http requests and the Elasticsearch.js client: Elasticsearch.js client in the new platform #12442
• Types cleanup
  • (aka do NOT allow plugins to require into core, even for types)
  • Move types out of "local files" to prepare for external types. This is already done for a couple things, but not at a proper scale.
• Elasticsearch.js typings
  • They are quite bad and needs tons of improvement
  • Make headers allowed
  • client.close doesn't exist in the typings
  • I think we should fork the existing one into ./packages and let it evolve with the rest of the code base.
• Http server: [server] Expose HTTP service to plugins #12464
• Better Typescript dev+build setup, included in Webpack et al
  • How should we start the server? Right now I "hacked" a scripts/platform.js in there, just to have something.
  • There are still a couple requires in the codebase we need to remove. Some of them seem to be related to default export stuff.
  • Use import() instead of require for plugins? (It makes starting plugins async, which doesn't fit nicely with starting the http server now, so we might need to await or something like that)
• Logging: [Meta] Logging in the new platform #12469
• eslint/tslint: TSLint/ESLint in the new platform #12470
• Directory structure: Directory structure with the new platform #12466
• Collect coverage from platform/plugins
• Improve error handling
  • REST endpoints incl validation
  • Config errors at startup
  • Failure in long-running observables
• Maybe explore having "local readmes" that explain the different parts of the system? E.g. in ./logger we have a readme that explains how the logger works. There is an example for the [plugin system][./server/plugins/README.md].

[kimjoar]

The build setup is bad right now, and this needs a lint setup, better TS setup, etc.

[epixa]

I get an npm error when I try to build this with a fresh npm install:

courtmbp:kibana Court$ npm run ts:build

> kibana@6.0.0-alpha1 ts:build /p/kibana
> tsc

platform/server/elasticsearch/Cluster.ts(24,17): error TS2339: Property 'close' does not exist on type 'Client'.
platform/server/elasticsearch/Cluster.ts(25,23): error TS2339: Property 'close' does not exist on type 'Client'.
platform/server/http/SslConfig.ts(36,26): error TS2339: Property 'constants' does not exist on type 'typeof "crypto"'.

[kimjoar]

This was merged into the new-platform feature branch, where we'll be working more on this before preparing a proper PR to master.