Add NamespaceSelect component

[david-martin]

example usage:

import NamespaceSelect from './namespace/NamespaceSelect';

// ...

const MyPage: React.FC = () => {
  const [selectedNamespace, setSelectedNamespace] = React.useState('');

// ...

  return (
    <NamespaceSelect selectedNamespace={selectedNamespace} onChange={setSelectedNamespace} />
  );
};

[R-Lawton]

lgtm works as expected