[EuiMarkdownEditor] — Allow external control of viewMode for Markdown editor and allow not displaying editor/preview button

[mbondyra]

We’re planning to implement the markdown visualization and here are the new designs.

As you can see, some parts aren’t achievable with the current markdown component. The changes we’d need are:

1. Ability to hide the preview/editor toggle from the toolbar’s right side
2. In the same spot, be able to display a help button instead
3. Ability to display a customFooterComponent

Here's the proposed API:

customFooterContent?: Component;
toolbarRightContentType?: 'previewToggle' | 'help';

Open to better ideas too!