Allow date picker to have relative dates to start/end rather than 'now'

[stevedearl]

This may be a bit of an edge case, but it would be useful for us to be able to select an absolute start date, and then have a relative date of +24h from that start date.
The reverse may also be useful, so that we set an absolute end date and have the ability to set the start date relative to it, say -7 days.

At the moment, we can only set dates relative to 'now'.

Describe a specific use case for the feature:
We monitor error logs from our applications on a 24 hour cycle from 8am-8am every day.
At the moment we need to set both 'start' and 'end' date to absolute values in the datepicker to set the correct timespan. It would be handy to be able to set 'end' date to 8am today and 'start' date to -24h relative to end date.
This is not a major feature by any means but I thought I'd raise it...!

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[henrikno]

I don't think this is an edge case, I think this should be the default when picking a date and clicking relative.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

We are closing this issue in order to provide better transparency of priorities. This issue won't be prioritized in the near future. We track it in #163011 for long term planning.