[5.x] Add create_blueprints config to entries fieldtype

[el-schneider]

This adds a create_blueprints config option to the entries fieldtype, allowing you to restrict which blueprints appear in the "Create & Link" dropdown.

This is the creation-side counterpart to the blueprints config added in #7047, which filters the selection list. It follows the same approach: a YAML-only config, not exposed in the CP field configurator, consistent with how blueprints was introduced.

Usage

related_entries:
  type: entries
  collections:
    - pages
  blueprints:          # filters selection list (existing, #7047)
    - article
    - page
  create_blueprints:   # filters creation dropdown (new)
    - article

[jasonvarga]

We're not adding new features to 5.x. Please target 6.x. Also, I think the existing blueprints config should probably be used for that dropdown. I don't think we need a new setting.

[el-schneider]

It is currently used for that, the idea is to have different configs for creation and linking. Our use case is, that we need our users to be able to link to everything, but only create certain entries.

[jasonvarga]

If you mark a blueprint as hidden I believe it gets excluded from creation dropdowns.