What does this PR do?

Adds theme.datatable.body.row.extend so that styles can be added to rows within the DataTable body. This enhancement allows for changes like removing the bottom border of the last row in the body of the datatable so we don't get a double border when we have a footer or pagination.

This can be achieved with:

dataTable: {
  body: {
    row: {
      extend: `&:last-child td {
        border: none;
        }
        &:last-child th {
        border: none;
        }`,
    },
  },
},

Where should the reviewer start?

What testing has been done on this PR?

Added jest test and modified custom theme datatable story

How should this be manually tested?

Do Jest tests follow these best practices?

• screen is used for querying.
• The correct query is used. (Refer to this list of queries)
• asFragment() is used for snapshot testing.

Any background context you want to provide?

What are the relevant issues?

Closes #7476

Screenshots (if appropriate)

Do the grommet docs need to be updated?

yes

Should this PR be mentioned in the release notes?

yes

Is this change backwards compatible or is it a breaking change?

backwards compatible