Allow unmanaged autogenerated sequence names

[matthewtusker]

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

[matthewtusker]

A bit of backstory here: oracle-enhanced was used at my current workplace until #1669 was implemented, which dropped support for trigger-based primary keys. This broke the app using it, and a fork was made with this change to keep things running. The database is managed outside of Rails so we don't care about schema dumps containing information about triggers (which was previously implemented), we just want to ignore the next_sequence_value as was being done before.

This isn't my code, I'm just trying to upstream it to prevent us having to maintain a fork with a single unique commit. We're also using the release61 branch because we're on Rails 6.1, but we want (well, need) to upgrade Rails to 7, which would be much easier if this change was included here (and backmerged as appropriate).

I'm also aware that this change is untested. I can look into adding these but may need some advice on the best place to add them.

[emm26]

My workplace could also benefit from the changes in this PR being merged in!