chore(boards2): Make public API non-crossing again, move to main folder

[jefft0]

boards2/v1/hub has simplified structs and API queries for read-only access to boards2 data. But the interrealm changes in #5669 required adding cur realm to all the hub functions in order to authorize the calling realm. This made them crossing functions and not suitable for a query API. Therefore, we must give up the "hub" abstraction.

In general, this PR moves the simplified structs to p/gnoland/boards/exts/hub and the public API in hub.gno to the main folder.

• Move the definitions of Board, Thread and Comment (and related tests) to p/gnoland/boards/exts/hub. Also move the local helper format.gno to there
• Move the public API in hub.gno to the main folder. Access gBoards directly instead of GetBoard, etc. in protected.gno. Rename it's helper getThread to getBoardThread, and have it call the original getThread directly
• Remove unused protected.gno . This includes GetRealmPermissions and GetBoard which check permissions (no longer supported). So remove the related filetests
• Rename the tests in boards2/v1/hub/filetests from z_*.gno to z_hub_*.gno . Move them to boards2/v1/filetests
• The hub folder is now empty, so remove it

TODO: Check if should remove ref *boards.Board from the Board struct, etc.
TODO: Confirm the updated header comment in hub.gno .

[Gno2D2]

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