[TabLayout] Ability to change the textAppearance based on the selection state of a Tab

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**Is your feature request related to a problem? Please describe.**
We have a requirement that wants us to be able to change the textappearance of a tab based on it's selection state. We can change text color as it uses a state selector to manage this, but are unable to change the fonts, letter spacing, or text size by setting a selector.

The only way seems to be to do this programatically instead of being able to style these changes.

**Describe the solution you'd like**

it would be nice to have something like **textAppearanceUnselected** or something similar that would allow setting the textAppearance of a tab when it is in an unselected state. The regular textAppearance attribute can be used for the selected state or if there isn't an unselected attribute set.

**Describe alternatives you've considered**

We are considering listening to the tab selection events, and adjusting, but this may not necessarily work as well as it can't be driven by a Theme Overlay in which case the states could be changed based on business rules that cause the theme to change as well.

Start by locating the TabLayout implementation and its tab text styling and selection-state handling. Review how the existing textAppearance and stateful text color behavior are applied. Done means unselected tabs can receive a separate text appearance while the regular appearance remains the selected-state fallback, with appropriate coverage for the new behavior.

Written by the indexing model from the issue text.