feat: Add a new `readonly` property to Model fields definition

readonly will be a boolean property by default set to false.

Its operation can be something like:

• when an instance is created or updated, if the field haven’t a value yet, then can be set without triggering a readonly error.
• once the field has a value, any further modification attempt will trigger a readonly error.

This come in handy when manipulating this kind of fields, which is a common use case I think (I’ve the need of this kind of model fields across several different projects).

So, what do you think?

[reinert]

This definetly should be in core. ReadOnly option is a must have for ORMs.

[leodutra]

Please consider it as core feature, gentlemen.