Implement XEP-0447: Stateless file sharing: File sharing element

[lnjX]

Implements parsing for the file sharing element from XEP-0447:
Stateless files sharing version 0.2.

https://xmpp.org/extensions/xep-0447.html

PR check list:

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

[codecov]

Codecov Report

Merging #448 (dcf67ce) into master (321a5d9) will increase coverage by 0.89%.
The diff coverage is 68.33%.

❗ Current head dcf67ce differs from pull request most recent head 7779427. Consider uploading reports for the commit 7779427 to get more accurate results

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   68.15%   69.05%   +0.89%     
==========================================
  Files         271      269       -2     
  Lines       23822    23684     -138     
==========================================
+ Hits        16235    16354     +119     
+ Misses       7587     7330     -257     

Impacted Files	Coverage Δ
...mple_2_rosterHandling/example_2_rosterHandling.cpp	0.00% <0.00%> (ø)
..._3_transferHandling/example_3_transferHandling.cpp	0.00% <0.00%> (ø)
examples/example_6_rpcClient/rpcClient.cpp	0.00% <0.00%> (ø)
...le_7_archiveHandling/example_7_archiveHandling.cpp	0.00% <0.00%> (ø)
examples/example_9_vCard/example_9_vCard.cpp	0.00% <0.00%> (ø)
src/base/QXmppBookmarkSet.cpp	0.00% <0.00%> (ø)
src/base/QXmppHttpUploadIq.cpp	100.00% <ø> (ø)
src/base/QXmppMessage.h	100.00% <ø> (ø)
src/base/QXmppPubSubNodeConfig.cpp	49.66% <0.00%> (ø)
src/client/QXmppArchiveManager.cpp	0.00% <0.00%> (ø)
... and 164 more

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