Add BRT support to zpool prefetch command

[amotin]

Implement BRT (Block Reference Table) prefetch functionality similar to existing DDT prefetch. This allows preloading BRT metadata into ARC to improve performance for block cloning operations and frees of earlier cloned blocks.

Make -t parameter optional. When omitted, prefetch all supported metadata types (both DDT and BRT now).

How Has This Been Tested?

Manually from command line, prefetching the BRT and observing dramatically better delete performance for large cloned files after.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Looks good to me, nice and straight forward.