Expose shared tooltip/crosshair's timestamp as variable

[zorba128]

Why is this needed:

I'm looking for way to link table view with chart, so that table shows some details
on point currently selected.

Now crosshair is shared only among charts, and this feature would allow implementation
of nice interactive details views, such as:

• show me some additional stats on point hovered
• show log entries from time where cursor is
• jump to some specific dashboard located at specific point in time

Maybe this could also become base for customizable graph tooltips (run some queries against given point rather than just displaying chart value).

What would you like to be added:

I believe simply exposing shared crosshair's timestamp as variable would allow implementation of such behaviors using existing features (utilize @ modifier in promql). It will require some manual coding (compared to some mechanism to auto-configure query range based on crosshair position), but it seems to already make things possible, while being easy to introduce and non-breaking.

[Add a brief description of what the feature or enhancement should do.]

Add {{crosshair_timestamp}} variable that would point at current crosshair position, or end of dashboard's time range.

[leeoniya]

see #78392