Allow filtering by blueprints in Entries fieldtype

[FrittenKeeZ]

I've had the need to do this for all projects we've done, so now I've decided to make a PR for it instead of just making a custom fieldtype extending entries.

[what-the-diff]

• Added a new if statement to the getIndexItems method.
• The condition checks for an array of blueprints in the field's configuration and adds them as filters to the query, if they exist.