feat: deprecated attributes

[caspiano]

add a default false deprecated option to attribute.
access to a deprecated field will be logged as a warning.

class Model < ActiveModel::Model
  attribute shiver_me_timbers : String?
  attribute shine_my_timbs : String?, deprecated: true
end

Model.new.shine_my_timbs
# => WARN source=active-model message=deprecated field `shine_my_timbs` accessed

[kimburgess]

An alternative to the additional arg is using the deprecated annotation from stdlib. This looks to be safe to use here - when the marco expands it will associated with the instance var so won’t fill compiler warning with garbage. The advantage of this is it keeps the same semantics as deprecating other functionality.

[caspiano]

Logging access at runtime if the annotation is present seems easy enough