Add Column.strftime/strptime instead of overloading as_string/datetime/timedelta_column

[mroeschke]

Description

Column.as_string/datetime/timedelta_column had a format argument that was not used for columns that weren't these types or didn't require conversion to these types.

This PR introduces a strftime and strptime on the column that will handle this format argument.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

/merge