EA: Make validate_fill_value part of interface
We defined validate_fill_value for DTA/TDA/PA and I'm thinking it may be worth requiring more generally and using on `ExtensionBlock`.  In particular, `ExtensionBlock._can_hold_element` ATM unconditionally returns `True`.  _can_hold_element isn't well-documented, but my intuition as to what it _should_ mean more or less matches what _validate_fill_value means.