Added variants handling

[delvedor]

As titled.

External variant example:

/** @variants external */
type FooContainer = Faz | Bar

/** @variant name='faz' */
class Faz {
  faz: string
}

/** @variant name='bar' */
class Bar {
  bar: string
}

Internal variant example:

class TestPolymorphicBase {
  baz: boolean
}

class TestPolymorphic extends TestPolymorphicBase {
  type: 'heya'
}

enum Foo {
  hello = 1,
  ciao = 2
}

class TestPolymorphic2 extends TestPolymorphicBase {
  type: Foo
}

/** @variants internal tag='type' */
type TestPolymorphicType = TestPolymorphic | TestPolymorphic2

TODO:

• Docs!
• Update Property and TokenFilter to use the new variant utilities. (depends on Address feedback from Kibana #232)

[swallez]

Loving how we end up with something simple yet capturing all that is needed for strongly typed languages!

Added some comments to add a 3rd variant kind container.

[swallez]

This should be:

Suggested change

	variants?: ExternalTag
	variants?: Container

[swallez]

LGTM once https://github.com/elastic/elastic-client-generator/pull/234/files#r602496498 has been fixed. This is beautiful!

[delvedor]

This is good to go 🎉
I've fixed all *Property issues, there are still a few errors in indices.put_mapping, but not related to this.

[swallez]

LGTM (still found a typo)

Awesome stuff, I'll add the @variants annotations and additional model validations in a follow-up PR.