Allow custom unusable-space blocker item

[onebeastchris]

This change would allow the specifying of a custom item (aka via custom model data) by specifying the Bedrock custom identifier ( as suggested in #3512 (comment).)

A few changes/issues:

• since there doesn't appear to be a clean method to get item definitions by their bedrock identifier (there is one for java, not bedrock), a loop through all the items until the item with the Bedrock identifier is needed
• the namespace needs to be specified too; so something like
  unusable-space-block: "geyser_custom:my_item" would be needed.

Fixes #3512