Support timestamp fields

Problem/Motivation

I wanted to experiment with this module to display Log entities (provided by this module: https://www.drupal.org/project/log). Logs have a timestamp base field, but that doesn't appear to be an acceptable field type for this module to use.

https://git.drupalcode.org/project/log/-/blob/e238d5e1e1875351580d655289...

Notably, Logs also have a created and changed field, and those DO work... so I dug into the calendar_view module and found this line:

https://git.drupalcode.org/project/calendar_view/-/blob/0c85f6ee7e665010...

If I add timestamp to the array on that line, it works! :-D

Proposed resolution

Add timestamp as an allowed field type to this line:

Remaining tasks

Create a merge request.

User interface changes

None.

API changes

Allow timestamp fields to be used for events in the calendar.

Data model changes

None.