minInterval should be configured with fixed or calendar intervals for time

[markov00]

Is your feature request related to a problem? Please describe.
In reference to an issue discovered in Kibana-Discover elastic/kibana#52152 we should better describe and implement how minInterval works on the xDomain.
The specified issue refers to a specific use case where the consumer specify a customMinInterval greater than the computed one when rendering a bar-chart. This can happen in various cases and we should try to avoid throwing errors but gracefully adopt a valid interval.

Describe the solution you'd like
In this PR we applied a fix to the Discover histogram,
elastic/kibana#52758
we should allow to specify various type of intervals: fixed or calendar (in case of time scales)

Describe alternatives you've considered
The minInterval is used to determine the bar width of a barchart in a linear x scale. We internally compute it by computing the difference between consecutive x values, but if the data is already scattered across the time range with many undefined points we can't compute a real interval automatically.

Additional context
In Kibana 7.6 and master we should add the test applied and available only on 7.5.1

Kibana Cross Issues
elastic/kibana#52152

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present