Make getDirtyAttributes detect updated dynamic fields

[djagya]

Or at least just mark the field we use for dynamic fields as 'dirty'.

It will be useful in many places as for example in TimestampBehavior, that updates timestamps only if model has dirty attributes.

What do you think @tom-- ?

[tom--]

I like the simpler option "mark the field we use for dynamic fields as 'dirty'."

The more specific case (distinguishing dirty and clean dynamic attributes in the tree of dyn attrs) seems complicated to implement and I'm not sure what value it has since MariaDB does not have a COLUMN_UPDATE() function and we don't use COLUMN_ADD to update individual dyn attrs.

[djagya]

@tom-- sounds good, I'll handle that