Add time column type

[benedictleejh]

Describe the problem to be solved

Grist only allows dates and datetimes for date/time related information. This would be unable to handle things like a recurring event happening at some specific time of day, where the date information is not relevant. Attempting to emulate this with datetimes is unsatisfactory currently as it is impossible to hide the date information for a datetime column.

A time data type is also standard in all major databases, and both Excel and Google Sheets easily display times without a day.

#1071 only requests for a duration column type, which is the difference between 2 datetimes, while this request is for a time column type, representing a specific time of day.

Describe the solution you would like

Implement a time column type. Python already has a time object available in the standard library, so maybe that could help with simplifying some implementation.