Add HttpUploadManager

[lnjX]

• Remove empty handleStanza() implementations
• Add QXmppError holding a description and std::any
• Add HttpUploadManager for requesting slots and uploading files

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
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #452 (860640c) into master (8afdd46) will increase coverage by 0.01%. The diff coverage is n/a.

:exclamation: Current head 860640c differs from pull request most recent head c17f4dc. Consider uploading reports for the commit c17f4dc to get more accurate results

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
+ Coverage   68.55%   68.56%   +0.01%     
==========================================
  Files         267      267              
  Lines       23613    23607       -6     
==========================================
  Hits        16187    16187              
+ Misses       7426     7420       -6     

Impacted Files	Coverage Δ
src/client/QXmppAtmManager.cpp	98.96% <ø> (+1.00%)	:arrow_up:
src/client/QXmppAtmManager.h	100.00% <ø> (ø)
src/client/QXmppAttentionManager.cpp	83.33% <ø> (+2.68%)	:arrow_up:
src/client/QXmppAttentionManager.h	100.00% <ø> (ø)
src/client/QXmppTrustManager.cpp	66.66% <ø> (+2.66%)	:arrow_up:
src/client/QXmppTrustManager.h	100.00% <ø> (ø)

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