[IMP] resource: add resource mixin adding fields and an API for resource

Purpose is to replace existing inherits of resource.resource by an inherit
on the mixin itself. The mixin offers several methods to compute working
intervals, attendance, hours, ... This mixin will be improved in future
commits when new methods that will be required with future improvements
of various service apps.

This commit also adds tests for the mixin itself. A new model is defined
only for testing purpose to test the mixin.

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