Implement custom preloads - backport to 4-x-stable

[apauly]

This PR backports the recently added "custom preloads" feature to the 4-x-stable branch.

In addition to a cherry pick, it required a couple of other updates:

• Adjust circle ci config as the docker images references on that branch are not available anymore, so I started from the current master and just replaced the workflows to match the old branch
• Use older sqlite version for ruby 2.6
• avoid using GlobalID#locate_many during specs

[jturkel]

Thanks for resurrecting the broken builds on this branch. A few minor comments on the build setup but the core of the backport looks good.

[jturkel]

We lint against the oldest supported version of Ruby. Can you get the lint job working against ruby:2.6.10?

[jturkel]

Can this install of SQLite be removed if we're pinning the gem to a corresponding older version?

[jturkel]

The gemfiles/*.gemfile files are generated by appraisals. Can you update the Appraisals file so these gemfiles can be regenerated via appraisal generate?

[apauly]

I tried to do that with the install_if option, but that failed for older ruby versions.
Can you give me a hint on where appraisal generate is actually being evaluated? It seems this is not during the CI build but locally before committing, so the conditional checking for the RUBY_VERSION cannot go into the Appraisals file, right? Or am I missing something?

[jturkel]

Correct we run appraisal generate locally and and checkin the generated gemfiles whenever we modify Appraisals. Leveraging the install_if option in Appraisals looks like it should work.

[apauly]

Hi @jturkel,
thanks again for your feedback!

Unfortunately I didn't find the time yet to continue working on this.
As my/our need for a backport of this feature was fixed since the initial PR, I will probably not be able to find the time continue working on this.

I'd be cool if the changes might be helpful when somebody else is looking for a backport, but for now, I'm closing the PR.