Added autoFocus option to EuiTabbedContent

[cchaos]

The focus state of EuiTabs can confuse users since it's so prominent compared to the selected state. Some users thing the focus state is the selected state. This PR attempts to allow setting the initial focus of the tabbed items from initial (first tab) to selected (selected tab).

There are 2 issues with it at the moment:

• [ ] 1. If the selected tab is not the first, users have to shift + tab in order to get to the previous tabs. Got confirmation from most that this is not an issue.
• 2. I'm currently using document.getElementById() to find the selected tab, which probably isn't great
• 3. The way I have it setup is that when a user focuses in on the EuiTabbedContent, it sets the state.focus to true and then auto focuses on the selected tab. The problem is that I have to reset the state to false when a user tabs out of the control. However, it seems to cycle as blur then focus each tab press.

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Also fixes the second action item for the EuiDatePicker from GAH:

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

[cchaos]

Opening this up for review. The "issue" item, is still unchecked mainly to see if anyone hollers about any concern over it. Otherwise, it's not really an issue and this should be good to go.

[thompsongl]

Code looks good; tested locally, too.

I don't think the shift + tab thing is a problem at all. The focus state is clear enough and that is the normal interaction for reverse focus order

[cchaos]

I'm merging this one in because I want to make sure it gets into K7.3