[FEATURE] Add horizontal crosshair option to TimeSeriesChart plugin

[oschrenk]

Closes perses/perses#4434.

Adds an optional axisPointer setting to the TimeSeriesChart tooltip spec to control the axis pointer of the ECharts tooltip. Specifically the single option to add a horizontal line.

With type: "cross", hovering a panel now shows a horizontal line at the cursor's y position in addition to the default vertical line. Useful for visually comparing series levels within a panel.

It adds a spec field (no UI editing as of now) mirroring the ECharts naming and nesting.

kind: TimeSeriesChart
spec:
  tooltip:
    axisPointer:
      type: "cross"

Choices:

• Single panel only. Currently this setting is enabled per chart and the horizontal line is kept active only on the hovered chart (no syncing across charts). Syncing would be misleading: ECharts connect forwards the y pointer by value (axisTrigger.ts#L407-L411), so other panels would show a line at the same y value mapped into their own scale, which means nothing across different units. This is the most controversial change and I introduced isHovered to distinguish a panel being hovered from the tooltip being shown (which can stay pinned). Open to other solutions here.
• cross only. While ECharts also allows line, shadow and none, only cross is accepted for now. Not sure if there is value in adding the other options, but it leaves room for it.
• No axis value labels. ECharts defaults label.show to true for cross (modelHelper.ts#L266-L269). I disabled it since the custom tooltip from Perses clashes with that idea.
• Stacked bar excluded. cross does not apply there.

Testing

• Manually tested live against a Perses v0.54.0 server with the plugin dev server, by rebasing the patch onto timeserieschart/v0.13.0 for host compatibility

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention.
• All commits have DCO signoffs.

[oschrenk]

As mentioned in the description. I distinguish hovered and showing tooltips because, tooltips might be pinned, but i might not hover anymore.

Please advice if that distinction make sense to you.