Allow disallowed_operations to be altered/extended

I totally agree something is needed here. Using hook_entity_extra_field_info() could be a way to help with this - whether in combination with this proposed hook_rabbit_hole_disallowed_operations_alter() hook, or instead of it.

Even so - the patch works quite well enough for my needs, even if doesn't allow for the easy configurability for non-coders that hook_entity_extra_field_info() would.

Setting to RTBC as the patch works well and is a worthwhile improvement over not having it. Further work could always be done in a follow-up if really wanted to make it more easily configurable.

@james.williams I appreciate your feedback, but I must mention that 8.x-1.x won't include new features. I'll make sure it's described on the module page.

In the meantime, feel free to take some time and test a new 2.0.0 version that should not have this issue.
Here is a change record with all major changes in 2.0.x: https://www.drupal.org/node/3359194 →

Fair enough :) Here's a rerolled patch for 2.0.x then, though it will need someone else to review & test now.