[EuiCodeBlock] Add new annotations feature

[elizabetdev]

The idea for this new feature is to highlight specific lines in EuiCodeBlock and provide additional information in a EuiPopover.

Visually it should look like the following:

Feature spec

The feature spec can be found here.

Notes

For the initial version, I think we should restrict the icon usage. The purpose of a EuiAnnotation is to highlight specific lines in an editor and provide additional information. So the icon should be indicative of this.