[Connector Builder] Set default value for end_datetime

[lmossman]

What

Resolves #21572

How

Sets the default value of end_datetime to {{ now_utc() }} using the existing BuilderOneOf default value mechanism.

[flash1293]

This behaves as discussed but I'm just wondering... this only works if the user also sets the date format on the slicer correctly which is what is used for the end datetime, right? Should we set it to the format now_utc() generates as well?

Otherwise it's actually quite hard to figure out what to do about it - I had to look through existing manifests to figure out that it's %Y-%m-%d %H:%M:%S.%f+00:00

If you don't set it, the config won't work

[lmossman]

@flash1293 I think your suggestion makes sense; setting the format will help people who don't want to change anything here, and will also give an example of a format that can be used. Added.

[flash1293]

LGTM