Is your feature request related to a problem? Please describe.
We need to display annotations on charts like the current time marker, partial-data areas or other general annotations.

Describe the solution you'd like
Add the possibility to include vertical or horizontal lines or rectangles on the chart, specifying the base x or y value where the line will lie.

This can be split into 3 different tasks:

• add line annotation + tooltip + marker Add annotation line, tooltip, and marker #104
• add rectangle annotation + tooltip Add rectangular annotation & tooltip #105
• possibly add text annotation Add text annotation  #106

Describe alternatives you've considered
n/a

Additional context
n/a

Kibana Cross Issues
Add any Kibana related issues/feature requests here.

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present