Extend as.Date Rapids expression to support conversion from timestamp

Columns representing timestamp as numeric or string (unix epoch) can not be easily converted to {{time}} type.

Suggestion is to extend the {{as_date}} (Py, R) methods and {{AstAsDate}} rapids expression to support this “epoch” time format, for example:

{code:python}col.as_date(format=“unix”)
# or
col.as_date(format=“epoch”){code}

Need to agree on format.