Add headerRow to EuiBasicTable

[andreadelrio]

Summary

This PR adds a new prop called headerRow to BasicTable that lets consumers specify what column is the row header (and thus should have <th scope=“row”>).

Doing something like

    <EuiBasicTable
      items={items}
      rowHeader="firstName"
      columns={columns}
      rowProps={getRowProps}
      cellProps={getCellProps}
    />

results in this markup

Limitations:

This only works with FieldDataColumn as it has field as a required prop which is what I’m using for doing

(column as EuiTableFieldDataColumnType<T>).field === headerRow

Fixes #2335 #2337

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox

• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[andreadelrio]

@myasonik this is my first pass at trying to cover #2335. Let me know what you think.

[myasonik]

From an a11y perspective, I think this is all that's needed and is good to go 👍 🎉

[thompsongl]

One suggestion, otherwise LGTM!

[myasonik]

VoiceOver output is correct 🎉

[andreadelrio]

After checking with @myasonik this also covers #2337 as CJ had suggested here

The screen reader does "[row x out of total] [name of row]*, Select this row"
*identifying column used in rowHeader