New filter: customize $calendar_datetime object

[davefx]

Added new filter that enables the further customization of $calendar_datetime object, for example by using a derived custom class.

Problem this Pull Request solves

We'd like to further customize the generated $calendar_datetime object, adding data in the object initialization that links the event with data from other plugins. This data will be processed used by other filters in the method.

How has this been tested

It's just a new filter. It doesn't affect the current code base unless the filter is used as a new hook for a new callback.

Checklist

• [x] I have read the documentation relating to systems affected by this pull request, see https://github.com/eventespresso/event-espresso-core/tree/master/docs
• [x] User input is adequately validated and sanitized
• [x] all publicly displayed strings are internationalized (usually using esc_html__(), see https://codex.wordpress.org/I18n_for_WordPress_Developers)
• [x] My code is tested.
• [x] My code follows the Event Espresso code style.
• [ ] My code has proper inline documentation.

[davefx]

@eeteamcodebase @manzoorwanijk Hi! Would you consider merging this PR?