add support for calculating the hash value of files

[drakkan]

We implement the following custom FTP commands:

• XCRC (requests CRC32 digest/checksum)
• MD5/XMD5 (requests MD5 digest/checksum)
• XSHA/XSHA1 (requests SHA1 digest/checksum)
• XSHA256 (requests SHA256 digest/checksum)
• XSHA512 (requests SHA512 digest/checksum)

and we also support the more modern HASH command

https://tools.ietf.org/html/draft-bryan-ftpext-hash-02

To support partial hash for the HASH command we should add support for RANG too.

This implementation is very similar to proftpd mod_digest

[codecov]

Codecov Report

Merging #196 (c55f77c) into master (6f53c30) will increase coverage by 2.85%.
The diff coverage is 83.19%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   64.25%   67.10%   +2.85%     
==========================================
  Files           9        9              
  Lines         870      979     +109     
==========================================
+ Hits          559      657      +98     
- Misses        235      239       +4     
- Partials       76       83       +7     

Impacted Files	Coverage Δ
server.go	56.06% <ø> (ø)
handle_files.go	66.09% <74.02%> (+2.84%)	⬆️
client_handler.go	58.37% <100.00%> (+3.57%)	⬆️
handle_misc.go	92.17% <100.00%> (+8.65%)	⬆️

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[fclairamb]

It's good.

I think it's reasonable to do not enable it by default, it's indeed a good way to attack a server by consuming its CPU & bandwidth.

I might add an FS extension later so that it can directly bind to the stored checksum on the final backend. For example, Google Drive implemented with afero-gdrive supports fetching the MD5 of the file. Here that means performing an API call instead of download the file and performing a checksum on it.

[drakkan]

This is already disabled by default, I added the hasher interface too, thank you!