Support calculated fields for Loopback entities

[KalleV]

Is your feature request related to a problem? Please describe.

It would be great to add support for the Virtual data type for calculated fields:
https://sequelize.org/docs/v6/core-concepts/getters-setters-virtuals/#virtual-fields

This would also provide a big improvement over the limited support and workarounds for calculated fields when using the Loopback juggler ORM: loopbackio/loopback-next#2707

Describe the solution you'd like
Ability to configure the virtual data type for Loopback entities.

Describe alternatives you've considered
A workaround I found is to define a global Sequelize hook and then mutate the attribute definitions in a beforeDefine hook to switch them to the Sequelize virtual data type.