Expand on entity serialization API

[SoSeDiK]

Original: #8492.

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Improves upon entity serialization API.

• IllegalArgumentException will now be thrown if something goes wrong instead of silently failing and producing invalid data (documented in javadocs).
• Allows optionally serializing passengers and spawning entities with passengers on them.
• Adds a few flags to control behavior in places where serialization would normally fail.

Added serialization flags

• FORCE - serializing entities that wouldn't normally be serialized.
  • This covers non-persistent (Entity#isPersistent) and invalid (Entity#isValid; i.e. dead, despawned) entities.
• MISC - serializing misc non-saveable entities (lightning bolt, leash, fishing bobber).
• PASSENGERS - if present, the serialized data will include serialized passengers as well.
  • All passed flags apply to passengers too, i.e. you'd also need to pass FORCE if some passenger is currently non-serializable.
• PLAYER - if anyone for whatever reason wants to save/look at serialized player's data.
  • Special case since vanilla always skips entity with Player's id, so deserializing such data will lead to IllegalArgumentException.

• Also touches virtual entities (entity snapshots) and Entity#getAsString to not fail in cases described above (unlike serialization, I see no good reason to fail e.g. just because the entity is marked as non-persistent).

Things to consider:

1. There seems to be no way of checking whether the misc entity is non-saveable. Should something like EntityType#isSaveable be exposed in a separate PR?