Fix fiber support not loaded in open-telemetry/context

[Nevay]

Adds missing autoload entry for initialize_fiber_handler.php and adds logging if OTEL_PHP_FIBERS_ENABLED is enabled and initialization of fiber support fails.

[codecov]

Codecov Report

Merging #829 (b6ff0a9) into main (deeb07a) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #829      +/-   ##
============================================
- Coverage     82.54%   82.46%   -0.09%     
- Complexity     1877     1878       +1     
============================================
  Files           230      230              
  Lines          4888     4893       +5     
============================================
  Hits           4035     4035              
- Misses          853      858       +5     

Flag	Coverage Δ
7.4	82.47% <0.00%> (-0.09%)	⬇️
8.0	82.49% <0.00%> (-0.09%)	⬇️
8.1	82.49% <0.00%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
src/Context/ZendObserverFiber.php	0.00% <0.00%> (ø)

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