Add property inferred_type for Index & MultiIndex

[itholic]

Since Index.mixed is deprecated in pandas, and they recommend use inferred_type.

• Anyway in Koalas, is_mixed() is not that useful because we don't support the mixed type though.

>>> pd.Index([1, 2, 3]).is_mixed()
<stdin>:1: FutureWarning: Index.is_mixed is deprecated and will be removed in a future version. Check index.inferred_type directly instead.

>>> ks.Index([1, 2, 3]).inferred_type
'integer'

>>> ks.Index([1.0, 2.0, 3.0]).inferred_type
'floating'

>>> ks.Index(['a', 'b', 'c']).inferred_type
'string'

>>> ks.Index([True, False, True, False]).inferred_type
'boolean'