Allow dealerdirect/phpcodesniffer-composer-installer plugin

[jdreesen]

Fixes an error in the CI pipeline during composer install:

Error: dealerdirect/phpcodesniffer-composer-installer contains a Composer plugin which is blocked by your allow-plugins config. You may add it to the list if you consider it safe.
You can run "composer config --no-plugins allow-plugins.dealerdirect/phpcodesniffer-composer-installer [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)
See https://getcomposer.org/allow-plugins

In PluginManager.php line 744:
                                                                               
  dealerdirect/phpcodesniffer-composer-installer contains a Composer plugin w hich is blocked by your allow-plugins config. You may add it to the list if you consider it safe.                                                       
  You can run "composer config --no-plugins allow-plugins.dealerdirect/phpcodesniffer-composer-installer [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)                              
  See https://getcomposer.org/allow-plugins
                                    

It's a dependency of doctrine/coding-standard:

> composer why dealerdirect/phpcodesniffer-composer-installer
doctrine/coding-standard 8.2.1 requires dealerdirect/phpcodesniffer-composer-installer (^0.6.2 || ^0.7) 
slevomat/coding-standard 6.4.1 requires dealerdirect/phpcodesniffer-composer-installer (^0.6.2 || ^0.7) 

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Probably all pipelines will fail until this is resolved. See #894, #893.

[jdreesen]

Duplicate of #897