[Table] provide default aria-labels, fix accessibility violations listed in storybook

[tay1orjones]

Describe the bug

From Internal Watson-IoT/pal-tracking#338 from @JordanWSmith15

Graphite storybook: https://pages.github.ibm.com/maximo-app-framework/graphite/react/storybook/?path=/story/components-%E2%98%A2-table--datasource-table

In the Knobs section, at the bottom, enable the "View Manager Dropdown" checkbox Go to the "Accessibility" tab to scan Result: 4 violations These violations are not reproducible in the IoT data table, because they are caused by a combination of knobs that are not reproducible there. However, these issues cannot be fixed in Graphite, as they are coming from the IoT table

[tay1orjones]

My response from the internal issue:

It looks like the a11y errors change based on when you open the a11y tab. On initial render without interacting with the table, the following 2 items are listed:

1. Buttons must have discernible text
2. All cells in a table element that use the headers attribute must only refer to other cells of that same table

I see four items if I open the column picker first and then select the a11y addon tab. It looks like we're just not providing sensible defaults for some of these elements. Carbon provides props for much of the ARIA items. We'll need to dig in to make sure, but I think these are easily resolvable.

[tay1orjones]

We also can investigate the meaning of Critical, Serious, and Minor labels and how they correspond to WCAG AA vs AAA or similar.