Mixins to extend UserModifiableEntity and BaseEntity classes

[akshatdubeysf]

Is your feature request related to a problem? Please describe. As of now, the @sourceloop core package provides a two models - UserModifiableEntity and BaseEntity that can be extended by the user, but the properties in these models are not configurable, exporting them as Mixin would provide an option to make them configurable, as well as Working with other base classes.

Describe the solution you'd like These Models should be available as Mixins.