Dashboard: Add customizable point-in-time markers for time-based line charts

## What?
Similar to #22997 (which essentially adds the ability to include horizontal lines on bar/line charts) it would be helpful to have the ability to add vertical lines at set times on time-based line charts. The user should be able to choose the color of the vertical line and also set a label.

## Why?
When investigating an issue (or preparing a report on an issue), it can be very helpful to have such annotations on graphs to mark the times when particular events happened, especially if those events are related to what the graph shows but don’t appear on the graphs themselves.

As an example, you have a graph showing total bytes sent and received by a host and you know from other log data that a compromised account logged into that host at 13:37 on Tuesday. You can put a red line on the network traffic graph at the time the compromised account logged into the box to help you contextualize the sudden rise in outbound traffic as likely data exfiltration.

For a more operations-focused use case, you may have a graph showing the number of errors raised by your application over time. Being able to add a vertical line to the graph marking the time of the most recent code deployment can be very helpful if you're trying to determine whether a sudden spike in errors is related to the deployment or not.

Something like this: