Add throttle_first flow op

[shariarriday]

Relates https://github.com/actor-framework/actor-framework/issues/1389

Scaffolded from Sample

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 15 lines in your changes missing coverage. Please review.

Project coverage is 72.17%. Comparing base (f83d342) to head (727e7ed). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
libcaf_core/caf/flow/op/throttle_first.hpp	82.55%	6 Missing and 9 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2097      +/-   ##
==========================================
- Coverage   72.21%   72.17%   -0.04%     
==========================================
  Files         662      663       +1     
  Lines       29625    29717      +92     
  Branches     3255     3267      +12     
==========================================
+ Hits        21393    21448      +55     
- Misses       6361     6382      +21     
- Partials     1871     1887      +16     

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