Fix NTP Authenticator parsing for non-MD5 digests (SHA1, SHA256, SHA512)

[Copilot]

NTP Authenticator fields were hardcoded to MD5's 16-byte digest size, causing misaligned parsing for SHA1/SHA256/SHA384/SHA512 MACs. The _NTPAuthenticatorPaddingField always assumed a 20-byte tail (4 key_id + 16 MD5 digest), so for e.g. SHA1 (24 bytes total), the first 4 bytes of digest were consumed as padding, key_id read digest bytes, and dgst was truncated.

• _NTPAuthenticatorPaddingField: Computes tail size dynamically by matching against known valid MAC sizes (20/24/36/52/68) instead of hardcoding _NTP_AUTH_MD5_TAIL_SIZE
• NTPAuthenticator.dgst: Changed from XStrFixedLenField with fixed 16-byte length to XStrField to consume all remaining bytes after key_id
• NTPExtPacketListField.getfield(): Same dynamic tail size detection when splitting extensions from MAC
• NTPHeader.guess_payload_class(): Added SHA384 (48-byte) digest to the recognized sizes
• NTPExtensions: _NTPAuthenticatorPaddingField also updated to use dynamic sizing via a new _ntp_auth_tail_size() helper

# Before: SHA1 authenticator parsed incorrectly
# padding='\x00\x00\x00\x0c', key_id=3523582088, dgst=<truncated 16 bytes>

# After: correctly parsed
# padding=None, key_id=12, dgst=<full 20-byte SHA1 digest>

Added tests for SHA1 (24-byte) and SHA256 (36-byte) NTP authenticator dissection and round-trip build/parse.

Original prompt

Check the 5 oldest issues. Are they still issues in current master?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.10%. Comparing base (ded1d73) to head (cdc5758).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4917   +/-   ##
=======================================
  Coverage   80.10%   80.10%           
=======================================
  Files         370      370           
  Lines       91727    91727           
=======================================
  Hits        73477    73477           
  Misses      18250    18250           

see 6 files with indirect coverage changes

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