Multiple connection support

[chaunce]

This implements support for multiple connections now native to Rails, as defined in guides https://guides.rubyonrails.org/active_record_multiple_databases.html

To add support without extensive branching logic I removed all Rails 5-specific code, this appears to align with the current state of the gem as specified in the changelog https://github.com/ilyakatz/data-migrate/blob/main/Changelog.md#900

Some modifications to the existing code were made to accommodate this support, some of this more closely matches the newest Rails schema migration code structure to allow :with_data rake tasks to function properly.

Tests were updated to match code changes, but a review of these to ensure they are still testing the intended functionality would be great. This is also light on specs specific to multiple connections, I can work on additional tests later.

[ilyakatz]

Hi @chaunce thank for making this PR, this is great. However, a couple of comments

1. looks like tests are failing for ruby 3.2
2. looks like you updated a bunch of things related to styling and such. It makes it difficult to see the real changes that were part of this PR. Is it possible to undo those stylistic changes and only include changes relevant for this PR? you can still make the style changes, but it's should be a separate PR.

thanks gain for the PR!

[chaunce]

Done, and done.

[ilyakatz]

Great, thanks for the changes. Since this is a pretty big refactor and it's a bit hard for me to validate it, I created another PR from your branch and made it so that it can push release candidates. First rc candidate is in rubygems now (see https://rubygems.org/gems/data_migrate/versions/10.0.0.rc1) so let's monitor it and see if anyone reports problems

[ilyakatz]

update, it's been running for a couple of weeks and no reported problems. i think i'll give it another week or 2 and after that we can take it out of rc

[ilyakatz]

This is now merged. Let's see if we get any issues in the coming days