[table]: add direction css class in sticky rows/columns

[shlomiassaf]

Bug, feature request, or proposal: proposal

What is the expected behavior?

When a sticky row/cell is set it add's a CSS (cdk-table-sticky, mat-table-sticky, etc...)
This should help with styling the row/column (e.g. adding borders...)

It works in simple use cases (1 sticky column)

But when using 2 (or more) sticky column pinned to start and 2 pinned to end it's hard to know find the edges of start/end (the last of "start" group, first of "end" group)

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