FormField required includes aria-required="true"

[halocline]

Please describe what the rule should do:

What category of rule is this? (place an "X" next to just one item)

[x] Warns about a potential error (problem)
[ ] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

<FormField
      htmlFor={id}
      name={name}
      label="Host Name"
      required
 >
 ...
 </FormField>

Why should this rule be included in eslint-plugin-grommet?

Ensuring proper aria attributes are applied. May be a nice enhancement to Grommet FormField?

[jcfilben]

Related to grommet/grommet#6524