EDR time axis compliance with coverageJSON

[pzaborowski]

Description
Current implementation of the XarrayEDRProvider generate time axis with label read from the source file into time_field with default value of 'time'. Time axes is defined by start, stop and steps.
But https://schemas.opengis.net/covjson/1.0/coveragejson.json
referred in https://docs.ogc.org/cs/21-069r2/21-069r2.html#_d5c16418-1a20-4dbf-bf7a-8e685062df97
expect 't' as time axis with 'values'.

This way it is implemented in https://github.com/Reading-eScience-Centre/leaflet-coverage to properly visualise coverage with >1 time slice (it does not serve other levels and start-stop definition).

Steps to Reproduce
Steps to reproduce the behavior:
open:
https://demo.pygeoapi.io/master/collections/icoads-sst/cube?f=html&bbox=-180,-90,180,90&datetime=2000-01-16/2000-03-17
select AIRT and get this

It looks like the template issue, which is partly true as it could be solved with changes in query template like in this commit:
f4f24f0
but these will not will not work with LeafletCoverage library without chaging time axis to t or changes in the JS libraries to support non-standard encoding.

Expected behavior
Coverage with >1 time value shall be presented as separate layers.
With current state it can be done as in the commit above but potentially breaking changes or changes in coveragejson schema and leaflet plugin. something in the middle is switch in the configuration that could render 'values' instead of 'start-stop-steps' but not sure if preferable. let me know< I can PR what is done already.