Support for additional dimension grouping in charts

[nahiyan-yasaar-selise]

Take this chart as an example:

It is possible that users would want values in each quarter to be grouped according to a different field, such as Account Manager. So the main dimension is Quarter, and the secondary dimension is Account Manager, so you end up with:

Q1
  AM1
    Metric 1
    Metric 2
  AM2
    M1
    M2
Q2
  AM1
    Metric 1
    Metric 2
  AM2
    M1
    M2

Also, similar to this request, allow arbitrary number of such dimension additions.

In terms of front end implementation for the builder, you can have an '+Add dimension' button similar to '+Add metric' which will let users set the subgroup. Adding another dimension make it the sub-subgroup.
Also allow for reordering dimensions similar to metrics. Reordering will change the hierarchy of dimension grouping.

Tying this feature together with this request, a non-primary dimension should have a checkbox "Make secondary axis". This will grey out the same checkbox for other dimensions to prevent weird behavior.

Additional UX feature:
Similar to the ability of clicking the legend hiding that metric in the chart block (in compose), users should be able to click on a group/subgroup to toggle this grouping in the client side.