Add verify command

[jonmchan]

Disclaimer - I probably wouldn't merge this code as is, but leaving it out there for reference in case anyone else wants it. I haven't written golang in years and the code is quite hacky.

I added a verify command that downloads all the chunks and verifies the SHA256 checksum.

Implementation wise, I cloned the receive command ripping out the zfs receive portion leaving only the SHA256 checksum checks. I added some messaging to show the results. It takes the same parameters as receive basically minus the destination dataset.

Example output:

success:

2024/12/19 19:31:08 spool/storage|backup_20241219193059.zstream.gz.pgp.vol1 - passed checksum!
2024/12/19 19:31:08 spool/storage|backup_20241219193059.zstream.gz.pgp.vol4 - passed checksum!
2024/12/19 19:31:09 spool/storage|backup_20241219193059.zstream.gz.pgp.vol3 - passed checksum!
2024/12/19 19:31:09 spool/storage|backup_20241219193059.zstream.gz.pgp.vol2 - passed checksum!
2024/12/19 19:31:09 Done. Elapsed Time: 354.358288ms
2024/12/19 19:31:09 Done - all backup chunks validated.

failure:

2024/12/19 19:30:13 spool/storage|backup_20241219190011.zstream.gz.pgp.vol4 - passed checksum!
2024/12/19 19:30:13 error trying to download file spool/storage|backup_20241219190011.zstream.gz.pgp.vol2 - SHA256 hash mismatch for spool/storage|backup_20241219190011.zstream.gz.pgp.vol2, got be72cbadf45d0693655b8cd022b06b6ee105191b8800816e052665148f84c3f3 but expected 2cdd7a410793404b89ab8e3996dbb982281b896e016e85b11ff574ec701f9554
2024/12/19 19:30:13 spool/storage|backup_20241219190011.zstream.gz.pgp.vol3 - passed checksum!
2024/12/19 19:30:13 spool/storage|backup_20241219190011.zstream.gz.pgp.vol1 - passed checksum!
2024/12/19 19:30:13 error trying to download file spool/storage|backup_20241219190011.zstream.gz.pgp.vol2 - SHA256 hash mismatch for spool/storage|backup_20241219190011.zstream.gz.pgp.vol2, got be72cbadf45d0693655b8cd022b06b6ee105191b8800816e052665148f84c3f3 but expected 2cdd7a410793404b89ab8e3996dbb982281b896e016e85b11ff574ec701f9554
2024/12/19 19:30:15 error trying to download file spool/storage|backup_20241219190011.zstream.gz.pgp.vol2 - SHA256 hash mismatch for spool/storage|backup_20241219190011.zstream.gz.pgp.vol2, got be72cbadf45d0693655b8cd022b06b6ee105191b8800816e052665148f84c3f3 but expected 2cdd7a410793404b89ab8e3996dbb982281b896e016e85b11ff574ec701f9554
2024/12/19 19:30:16 error trying to download file spool/storage|backup_20241219190011.zstream.gz.pgp.vol2 - SHA256 hash mismatch for spool/storage|backup_20241219190011.zstream.gz.pgp.vol2, got be72cbadf45d0693655b8cd022b06b6ee105191b8800816e052665148f84c3f3 but expected 2cdd7a410793404b89ab8e3996dbb982281b896e016e85b11ff574ec701f9554
2024/12/19 19:30:18 error trying to download file spool/storage|backup_20241219190011.zstream.gz.pgp.vol2 - SHA256 hash mismatch for spool/storage|backup_20241219190011.zstream.gz.pgp.vol2, got be72cbadf45d0693655b8cd022b06b6ee105191b8800816e052665148f84c3f3 but expected 2cdd7a410793404b89ab8e3996dbb982281b896e016e85b11ff574ec701f9554