Encapsulate the blacklight config

[jcoyne]

All values set on Blacklight::Engine.config are actually stored
in a class variable on Railtie::Configuration. So these pollute
that namespace. All other railties are encapsulating their configs
onto a single entry in that scope. This change makes Blacklight do
likewise.

This probably needs to be a major revision

[bess]

+1 but it seems like we should talk about when to merge this, if we think it's a major version increment. Would it make sense to mark this as a Blacklight 8 feature?
UPDATE: And I see that @mejackreed already tagged this for 8.x. Thanks!

[barmintor]

Now that the 8.0.0.alpha tag is in main, can we revive this PR?

[barmintor]

Rebasing for CI before merge.

[barmintor]

The OpenStruct linter rule was added in the interim of this PR being opened. There's three approaches available to us:

1. Use OpenStructWithHashAccess here, and remove it (or not) along with the rest of the usages of that class
2. Add to rubocop_todo
3. Disable the Style/OpenStructUse linter rule.

I think there's an argument for disabling the rule rather than adding yet another todo, but I'm also sympathetic to using OpenStructWithHashAccess and dealing with that (or not!) as a single task in the future.

[tpendragon]

rubocop/rubocop#10206 is where they added this, and it makes a good point that Ruby 3 basically says "quit it."

I'm for option 1, and actually removing that class one day (although I suspect it's gonna be hard :( )

[barmintor]

Sold @tpendragon. I'll merge after CI.

[cbeer]

@jcoyne or @barmintor, can you create a PR against the release-7.x with appropriate deprecations and a migration path for this change?

[jcoyne]

@cbeer I wouldn't know how to do that as Blacklight::Engine.config is pointing at the Railtie::Configuration. Since that's not a class we control, we can't add deprecations to it.

[barmintor]

It's true that it might be awkward, but I created #2607 to try to work through it @cbeer

[barmintor]

Ok, I have a proposed deprecation up in #2610

Spoiler: It was kind of awkward.