feat: add the ability to load selected entrypoints

[nlemoine]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Add the ability to load a selected entrypoints
• Add BudEntrypointsLoader

What is the current behavior? (You can also link to an open issue here)

The EncoreEntrypointsLoader actually loads all assets found in entrypoints.json but this might result in loading non wanted assets if you don't (or can't) adhere to the location resolver naming convention.

What is the new behavior (if this is a feature change)?

Add an (array) $entrypoints parameter to the load method so you can manually set the desired entrypoints.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

The Loader interface remains unchanged so this should not.

Other information:

I also added a BudEntrypointsLoader which is basically similar to EncoreEntrypointsLoader besides it does not have an entrypoints key.