Allow for arbitrary entity placing logic similar to block placement

Prerequisites

• I am running the latest alpha version of Structurize for my Minecraft version.
• I checked the MineColonies/Structurize wiki and made sure that my issue is not covered there.
• I made sure that this change request is not a duplicate of any existing change request.

Context

• Minecraft Version: 1.16.5
• Structurize Version: 0.13.174-ALPHA

Is your feature request related to a problem?

Placing entities in non creative mode using Structurize is currently very limited. In ldtteam/minecolonies-features#447 I would like a way to have a book and quill or written book with custom text placed inside of an item frame.

I as I understand the code this is not possible given the way that Structurize is currently coded.

Enhancement or Change description

A system similar to the IPlacementHandler block placement system, but for entity placement would greatly enhance the flexibility of entity placement in mods using Structurize.

This would allow the creation of an entity handler that would consume a blank book and quill and place a item frame containing a book and quill with the custom text.

Notes or related things

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Start by reading the existing IPlacementHandler block-placement system and locating Structurize's entity-placement path. Define how an entity handler would receive placement context and consume or modify items, using the item-frame book-and-quill scenario as the acceptance case. Done means arbitrary entity placement handlers can support this scenario without creative mode.