[Grid] Spanned cells can be indistinguishable

[dtopalov]

Is your feature request related to a problem? Please describe.
Related to https://github.com/telerik/kendo-angular-private/issues/3345

The Grid rows/cells have no bottom/top borders, and the first cells in a row don't have left borders. This is perfectly fine in normal use cases, but causes the following problems when there are rows with rowspan > 1 (and thus - missing cells in the respectie column in the following N rows):

1. When consecutive spanned cells have the same background (depending on the odd or even index of their parent row), it is not clear where one cell ends and the other begins, and also which rows the cell spans through:
   https://dojo.telerik.com/MvbHYpXY
   

2. The left border of cells that happen to become the first TD child in their row (due to other configuration and the rowspan of the cell above) is also not rendered:
   

   .k-grid .k-table-th:first-child, .k-grid td:first-child, .k-grid .k-table-td:first-child {
       border-inline-start-width: 0;
   }
   

   This problem is mitigated in jQuery by rendering the "hidden" cells, but the correct approach is to not render the TD at all when the one above it has rowspan > 1.

Describe the solution you would like to see implemented
Spanned cells can receive the necessary borders (bottom/left/right) accordingly so that they are easily distinguishable from other cells, and it is clear what row they belong to, and how many rows they span. Additional adjustments would be probably needed to avoid double borders in edge cases.

Any other solution achieving the same goal and yielding a better look than the current one would also be fine.

Describe the alternatives you have considered
Until a good built-in default behavior is implemented, developers can achieve the desired look with custom styling (this approach can also can be used in documentation demos).

@inikolova @dmanova