Prevent propagator packages from throwing fatal error if sdk not installed

[Nevay]

$ composer init --require open-telemetry/extension-propagator-b3:^1.0 -n && composer u && php vendor/autoload.php

Fatal error: Uncaught Error: Class "OpenTelemetry\SDK\Registry" not found in /php/propagators-fatal-error/vendor/open-telemetry/extension-propagator-b3/_register.php:9
Stack trace:
#0 /php/propagators-fatal-error/vendor/composer/autoload_real.php(41): require()
#1 /php/propagators-fatal-error/vendor/composer/autoload_real.php(45): {closure}('2cea4639ba6ec5b...', '/php/propagator...')
#2 /php/propagators-fatal-error/vendor/autoload.php(25): ComposerAutoloaderInit35b4c2f542c1313891bfa1d737057525::getLoader()
#3 {main}
  thrown in /php/propagators-fatal-error/vendor/open-telemetry/extension-propagator-b3/_register.php on line 9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d414999) 84.08% compared to head (c4694f0) 84.08%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1198   +/-   ##
=========================================
  Coverage     84.08%   84.08%           
  Complexity     2237     2237           
=========================================
  Files           285      285           
  Lines          6359     6359           
=========================================
  Hits           5347     5347           
  Misses         1012     1012           

Flag	Coverage Δ
7.4	82.75% <ø> (ø)
8.0	84.01% <ø> (ø)
8.1	84.15% <ø> (ø)
8.2	84.15% <ø> (ø)
8.3	84.15% <ø> (ø)

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