[MCM-1215] Custom Spawn Eggs are broken when placed in chests

Partner Rank:
1

Verification Builds:
1.16.200.53_rc2 (4941996, W10)

Regression Builds: Map build in 1.16.40 W10 (4645976). Tested on 1.16.200.52_rc1 (4900268) and it worked.

Summary: Custom Spawn Eggs are broken when placed in chests - missing ItemIdentifier

Affects currently Live Content:
Yes (one in content review, one live piece that is de-listed at present)

Impact: Spawn Eggs lose their associated entity/actor ID. By observing the NBT data in the world before opening the world in RC2 it would appear that the ItemIdentifer NBT tag is inadvertently removed when loading the world in 1.16.200.53_rc2. This is impacting a piece of content in review and is also breaking a live piece of content.

Repro Steps:

Repro steps for the provided world (privately):

Load the world (DM'd)
Go to 4193 80 1001.
Interact with the NPC (right-click it).
Click "Get Cones" button.
Open the chest that appears nearby.
Notice the erroneous spawn eggs on the top row. See the screenshot of the chest UI below.

Observed Results: The spawn eggs should be set to `fs:cone`. At the moment, the spawn egg is not usable (i.e. it does not spawn entities when used). Please see notes section at the bottom of this bug report for extra insight.

Expected Results: The spawn eggs should have a title, icon, and be usable (i.e. you can spawn entities when you use the egg).

Screenshots/Videos: Yes (x2 videos), see DM to Greg Walls. Plus screenshots below:

https://i.gyazo.com/8f79de0037bf119dfd0271e7ea8a6b77.png

Notes: We also noticed some inconsistencies with spawn egg NBT data. Sometimes the game will assign an `ItemIdentifier` and call the entry `minecraft:spawn_egg` (as is the case here). But sometimes the game will not assign an `ItemIdentifier` and will instead use a different name. We made a new entity called fs:test_entity and this is what the NBT data looked like. https://i.gyazo.com/edfbcd18c450864190a407f6ffb88342.png - as you can see, the Name NBT tag is: `fs:test_entity_spawn_egg`. Whereas our content has `minecraft:spawn_egg` as the `Name` but has an additional `ItemIdentifier` tag.