Allow unmanaged autogenerated sequence names

To still support primary keys through db triggers, without adding back in
support for generating and maintaining them with migrations, add an
configuration boolen:

    unmanaged_autogenerated_sequences = false # default

Which can be used in an initializer file. When set to `true` there will not be
an exception raised when the sequence name is defined in the model with the
previously supported value:

   self.sequence_name = :autogenerated

This option allows for dbs managed outside of the application to use triggers
for primary keys. Without the `autogenerated` option an error is raised when
inserting the record:

    ORA-02289: sequence does not exist

lib/active_record/connection_adapters/oracle_enhanced_adapter.rb

@@ -212,6 +212,17 @@ class OracleEnhancedAdapter < AbstractAdapter
       cattr_accessor :default_sequence_start_value
       self.default_sequence_start_value = 1
 
+      ##
+      # :singleton-method:
+      # By default trigger based sequences are not supported.
+      # Enable this if you wish to manage your own primary key triggers, then you can avoid an
+      # exception being raised when using autogenerated sequence names in your ActiveRecord models:
+      #
+      #     self.sequence_name = :autogenerated
+      #
+      cattr_accessor :unmanaged_autogenerated_sequences
+      self.unmanaged_autogenerated_sequences = false
+
       ##
       # :singleton-method:
       # By default, OracleEnhanced adapter will use longer 128 bytes identifier
@@ -493,6 +504,7 @@ def discard!
       # called directly; used by ActiveRecord to get the next primary key value
       # when inserting a new database record (see #prefetch_primary_key?).
       def next_sequence_value(sequence_name)
+        return nil if unmanaged_autogenerated_sequences
         # if sequence_name is set to :autogenerated then it means that primary key will be populated by trigger
         raise ArgumentError.new "Trigger based primary key is not supported" if sequence_name == AUTOGENERATED_SEQUENCE_NAME
         # call directly connection method to avoid prepared statement which causes fetching of next sequence value twice