[5.x] Add support for collection entry blueprints loaded via addons

[xenco]

This PR enables addons to supply predefined blueprints for existing collections.

Closes statamic/ideas#1327.

Problem
Statamic previously introduced addon blueprint loading support (see #9324), but it was limited to top-level blueprints only. It did not allow addons to add blueprints to existing collections—for example, adding an extra blueprint to the "pages" collection to support features like a page builder with addon-provided and autoloaded fieldsets.

Solution
This PR extends the blueprint loading mechanism, enabling addons to load entry blueprints for any existing collection. To make use of this functionality, addons must follow this directory structure:

Addon Root
└── resources/
    └── blueprints/
        └── collections/
            └── {collection-handle}/
                └── {blueprint-name}.yaml

Which will result in the addon blueprints being visible and selectable for that collections entries.

[duncanmcclean]

It looks like this has caused a bunch of test failures. Would you be able to take a look?

Most of them are the same error:

[xenco]

I think the errors are caused by where I initially placed the changes. Although the modifications were only relevant for collections, they unintentionally affected all blueprints.
Additionally, the tests aren't configured yet to handle the new method addonEntryBlueprints in the Blueprint Facade.
I've now moved the logic directly into the collection and the tests run fine locally.

Sorry if this caused confusion or still is not the right way of doing things. It's my first time working with the core.
I hope my explanation makes sense and this change can be integrated into the codebase.

[duncanmcclean]

No worries!

[edalzell]

How is this different than the addon use the blueprint found event?

[xenco]

Hi @edalzell, if existing functionality already covers my use case, could you please provide me with an example?

Just registering the blueprints to a namespace does not work for me. I tried multiple variations of namespace and path. Still only the top-level blueprints are registered.

public function bootAddon()
{
    Blueprint::addNamespace("collections.pages", base_path('resources/blueprints/collections/pages'));
}