Add QXmppTrustManager as base of trust managers such as QXmppAtmManager

[melvo]

Before opening a pull-request please do:

• [ ] Documentation:
  • [x] Document every new public class and function
  • [x] Add \since QXmpp 1.X to newly added classes and functions
  • [x] Fix any doxygen warnings from your code (see log when building with -DBUILD_DOCUMENTATION=ON)
• [ ] When implementing or updating XEPs add it to doc/xep.doc
• [ ] Add unit tests for everything you've changed or added
• [x] On the top level, run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #423 (a7e3660) into master (2000431) will decrease coverage by 0.01%. The diff coverage is 89.15%.

:exclamation: Current head a7e3660 differs from pull request most recent head 075e572. Consider uploading reports for the commit 075e572 to get more accurate results

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
- Coverage   76.22%   76.21%   -0.02%     
==========================================
  Files         231      234       +3     
  Lines       20610    20672      +62     
==========================================
+ Hits        15711    15755      +44     
- Misses       4899     4917      +18     

Impacted Files	Coverage Δ
src/client/QXmppTrustMemoryStorage.h	100.00% <ø> (ø)
src/client/QXmppTrustStorage.h	100.00% <ø> (ø)
src/client/QXmppTrustManager.cpp	64.00% <64.00%> (ø)
src/client/QXmppAtmManager.cpp	97.93% <100.00%> (ø)
src/client/QXmppAtmManager.h	100.00% <100.00%> (ø)
src/client/QXmppTrustManager.h	100.00% <100.00%> (ø)
src/client/QXmppTrustMemoryStorage.cpp	100.00% <100.00%> (ø)
tests/qxmppatmmanager/tst_qxmppatmmanager.cpp	98.33% <100.00%> (ø)
tests/qxmppomemodata/tst_qxmppomemodata.cpp	98.25% <0.00%> (+<0.01%)	:arrow_up:

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