Implement XEP-0300: Use of Cryptographic Hash Functions in XMPP

[lnjX]

Parsing and serialization for XEP-0300 https://xmpp.org/extensions/xep-0300.html in version 1.0.

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/xep.doc
• [ ] Add unit tests (will follow in file sharing tests)
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #447 (a124198) into master (a9dc491) will decrease coverage by 0.23%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
- Coverage   69.28%   69.04%   -0.24%     
==========================================
  Files         261      263       +2     
  Lines       23362    23443      +81     
==========================================
  Hits        16187    16187              
- Misses       7175     7256      +81     

Impacted Files	Coverage Δ
src/base/QXmppHash.cpp	0.00% <0.00%> (ø)
src/base/QXmppHash.h	0.00% <0.00%> (ø)

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