caching_sha2_password can now be used without TLS

[anarthal]

close #313

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.15%. Comparing base (90405e7) to head (dfb962a).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #480   +/-   ##
========================================
  Coverage    99.15%   99.15%           
========================================
  Files          143      144    +1     
  Lines         7297     7370   +73     
========================================
+ Hits          7235     7308   +73     
  Misses          62       62           

Files with missing lines	Coverage Δ
include/boost/mysql/client_errc.hpp	100.00% <ø> (ø)
include/boost/mysql/impl/error_categories.ipp	98.80% <100.00%> (+0.02%)	⬆️
...sql/impl/internal/sansio/caching_sha2_password.hpp	100.00% <100.00%> (ø)
...l/impl/internal/sansio/csha2p_encrypt_password.hpp	100.00% <100.00%> (ø)
...ude/boost/mysql/impl/internal/sansio/handshake.hpp	99.31% <100.00%> (+0.08%)	⬆️
...sql/impl/internal/sansio/mysql_native_password.hpp	100.00% <100.00%> (ø)
include/boost/mysql/impl/is_fatal_error.ipp	100.00% <100.00%> (ø)
include/boost/mysql/impl/pfr.hpp	100.00% <ø> (ø)

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