Improve accessibility of EuiTableRowCell

[alisonelizabeth]

Currently, the EUI table does not provide a way to add a scope="row" attribute to a table cell (which as of HTML5 can only be applied to a <th> tag).

Screen readers provide the ability to navigate to individual columns randomly. Without a "scope=row", the screenreader user won't know what row a given cell is associated with. For more information, see: https://webaim.org/techniques/tables/data#headers.

Ideally, EUI would create a new component, or provide props on EuiTableRowCell that allow this capability.

This came up during an accessibility audit: elastic/kibana#43842

Additional discussion here: elastic/kibana#45024 (comment)