Add dt accessor to Index

[jbrockmendel]

Following discussion in #17134, this adds a dt property/accessor to pd.Index. This property should quack like pd.Series.dt. If index is not datetime-like, index.dt will raise an AttributeError. Otherwise it will return an object that is effectively index.to_series().dt

Subsumes parts of #17117

• closes DISC: add accessor attributes to Index for consistency with Series #17134
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jorisvandenbossche]

See my comment in the issue: #17134 (comment) which should be discussed first

[jbrockmendel]

This can be closed at least until #17134 is resolved.