Add initialPageIndex pagination prop to EuiInMemoryTable

[chandlerprall]

Summary

Resolves #1794

Adds initialPageIndex pagination prop to EuiInMemoryTable to allow specifying the initial page of results.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode

• Any props added have proper autodocs
  - [ ] Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
  - [ ] This was checked against keyboard-only and screenreader scenarios
  - [ ] This required updates to Framer X components

[chrisronline]

@chandlerprall I'm not sure this resolves #1274. That is more about parity between the pagination options available between EuiInMemoryTable and EuiBasicTable, notably pageIndex and pageSize. It'd be ideal if we could control these outside of EuiInMemoryTable (through our own state, triggered through onTableChange events) and we currently can't do it as we can't pass these props down.

[chandlerprall]

@chrisronline I'll remove #1274 from this PR then 👍

[thompsongl]

LGTM