Add combine_latest flow operator

[shariarriday]

Relates #1389

[codecov]

Codecov Report

Attention: Patch coverage is 80.19802% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 70.95%. Comparing base (cc33a98) to head (b49b60b).
Report is 10 commits behind head on master.

Files	Patch %	Lines
libcaf_core/caf/flow/op/combine_latest.hpp	78.88%	1 Missing and 18 partials ⚠️
libcaf_core/caf/flow/observable.hpp	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1809      +/-   ##
==========================================
- Coverage   70.99%   70.95%   -0.04%     
==========================================
  Files         654      653       -1     
  Lines       28377    28474      +97     
  Branches     3064     3082      +18     
==========================================
+ Hits        20145    20205      +60     
- Misses       6481     6506      +25     
- Partials     1751     1763      +12     

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