feat(Datagrid): add option to specify an initial sortable column

[matthewgallo]

Contributes to #4301

This PR adds the ability to specify a column to be sortable within the datagrid initial state, see following structure:

initialState: {
  sortableColumn: {
    id: 'firstName', // column id
    order: 'ASC' // sorting order, 'ASC' | 'DESC' included in docs update
  }
}

I created a custom hook called useInitialColumnSort that calls instance.toggleSortBy(), which comes from react-table, and is what triggers the sorting.

What did you change?

packages/ibm-products/src/components/Datagrid/Datagrid.docs-page.js
packages/ibm-products/src/components/Datagrid/Datagrid.stories.js
packages/ibm-products/src/components/Datagrid/Datagrid/DatagridHeaderRow.js
packages/ibm-products/src/components/Datagrid/useInitialColumnSort.js
packages/ibm-products/src/components/Datagrid/useSortableColumns.js

How did you test and verify your work?

Storybook

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