Simplify generics for IContextContainer

[joshdover]

Summary

This greatly simplifies the generic types for the IContextContainer by leveraging more type inference and conditional types to be able to pull out all the necessary types from a single generic.

Before

Before this change you had to separately define the type of the context object, the return type of the handlers, and any additional arguments provided to handlers.

core.context.createContextContainer<
  AppMountContext,
  AppUnmount | Promise<AppUnmount>,
  [AppMountParameters]
>();

After

Now, all you need to provide is the signature of the handlers:

core.context.createContextContainer<
  (
    context: AppMountContext,
    params: AppMountParameters
  ) => AppUnmount | Promise<AppUnmount>
>();

This can be simplified even further if you already have a type defined for this:

core.context.createContextContainer<App['mount']>();

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

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[joshdover]

@elasticmachine merge upstream

[mshustov]

ack will review tomorrow

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