Support for virtual fields from config

[r1tsuu]

There's a good blog post about Virtual Fields https://payloadcms.com/blog/learn-how-virtual-fields-can-help-solve-common-cms-challenges

And yes, it's true, they are great. But if you are using SQL database, the schema for this field is still created, because there's no way to determine whatever field is "virtual" or not.

My proposal would be to add a field option like:
isVirtual: true or db: { store: false } so then in our adapter we can skip the schema for this field, but instead we should validate for afterRead hook existance and automatically delete this field before create / update operation.