[MC-259358] The summoner of a vex is not stored to NBT

The owner/summoner of a vex is not stored to NBT, which means it is discarded when leaving and rejoining a world.

Summon an evoker
Go to survival mode
Run the following command to confirm that the final say command runs as the evoker
```
/execute if entity @e[type=vex] on origin run say hi
```
Leave and rejoin the world
Run the same command from 3. again and notice that it can't find the origin of the vexes.

Linked issues

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