[Connector builder] Allow format configuration for start/end time in dateslicer

[flash1293]

A lot of existing manifests use different formats for the start and end time in the date slicer component. Right now this can't be configured in the UI (it will always inherit the format from the slicing component itself which enforces consistent formats used in the API response and the configuration which can be problematic). It's possible to work around this by doing date formatting within jinja templates, but this case is common enough to provide it as a top-level UI element.

Acceptance criteria

• It's possible to specify an optional format only for the end date / start date of the date stream slicer component which is different than the main format
• The default behavior is still the same as before