Add allow_by_default option for forbid_lazy_load plugin

This one was conceived and discussed more thoroughly over in #2286. The
basic idea is that the `forbid_lazy_load` plugin disables lazy loading
on specific model instances, but also broadly on any models loaded
through an association. The latter can be detrimental when trying to cut
over to eager loading incrementally and where it's hard to get an entire
codebase converted all at once.

Here, the `forbid_lazy_load` plugin picks up a new `allow_by_default`
option. When it's set to true, lazy loading can still be forbidden
normally on an instance-by-instance basis, but it's not forbidden
through associations. This should make incremental migrations to eager
loading with `forbid_lazy_load` somewhat easier to do.

Author: brandur

CHANGELOG

@@ -6,6 +6,8 @@
 
 * Support treat_string_list_as_untyped_array Database option in pg_auto_parameterize_in_array extension (jeremyevans)
 
+* Support allow_by_default option in forbid_lazy_load extension (brandur) (#2288)
+
 === 5.90.0 (2025-03-01)
 
 * Avoid one query per enum when loading pg_enum extension when pg_array extension is already loaded (nick96) (#2278, #2279)

lib/sequel/plugins/forbid_lazy_load.rb

@@ -39,6 +39,9 @@ module Plugins
     #
     #   Album.first.artist # no error
     #
+    # This behavior of enabling +forbid_lazy_load+ automatically from dataset
+    # methods can be disabled using the plugin's +:allow_by_default+ option:
+    #
     # You can allow lazy loading associations for an instance that it
     # was previously forbidden for:
     # 
@@ -98,7 +101,17 @@ module Plugins
     #
     #   # Make the Album class support forbidding lazy load
     #   Album.plugin :forbid_lazy_load
+    #
+    #   # Let lazy loading be forbidden by object, but not automatically for any
+    #   # object loaded via dataset.
+    #   Album.plugin :forbid_lazy_load, allow_by_default: true
     module ForbidLazyLoad
+      def self.apply(model, opts=OPTS)
+        unless opts[:allow_by_default]
+          model.send(:dataset_extend, ForbidByDefault, :create_class_methods=>false)
+        end
+      end
+
       # Error raised when attempting to lazy load an association when
       # lazy loading has been forbidden.
       class Error < StandardError
@@ -179,7 +192,7 @@ def _load_associated_objects(opts, dynamic_opts=OPTS)
         end
       end
 
-      module DatasetMethods
+      module ForbidByDefault
         # Mark model instances retrieved in this call as forbidding lazy loading.
         def each
           if row_proc