Add post processor to run custom actions

[dontub]

Currently the new post processor can perform custom APIv4 calls. There can be made more types of actions possible through implementations of \Civi\Banking\PostProcessor\CustomAction\CustomActionHandlerInterface.

TODO: Documentation

systopia-reference: 32247

[dontub]

There's currently a phpstan error:

------ ------------------------------------------------------------------------------ 
  Line   CRM/Banking/PluginImpl/PostProcessor/MembershipExtension.php                  
 ------ ------------------------------------------------------------------------------ 
  458    Cannot access offset 'memberships' on array{memberships: list<mixed>}|false.  
         🪪  offsetAccess.nonOffsetAccessible                                          
 ------ ------------------------------------------------------------------------------

It's actually not a new issue. It should be checked, though I'm currently not sure what the intended behavior was when $result gets overridden with FALSE.

Another thing that I don't really understand: Strings saying a post processor might have been executed.

org.project60.banking/CRM/Banking/PluginModel/PostProcessor.php

Line 53 in 9cd4635

* NULL when it might have been executed.

org.project60.banking/CRM/Banking/PluginModel/PostProcessor.php

Line 68 in 9cd4635

return E::ts('%1 might have been executed.', [1 => $this->getName()]);

I'd suggest making \CRM_Banking_PluginModel_PostProcessor::shouldExecute() public and call it in \CRM_Banking_Matcher_Engine::runPostProcessors() before the call of \CRM_Banking_PluginModel_PostProcessor::processExecutedMatch()
and in \CRM_Banking_Matcher_Engine::previewPostProcessors() before the call of \CRM_Banking_PluginModel_PostProcessor::previewMatch(). (The latter should only be called if shouldExecute() returns TRUE.) If \CRM_Banking_PluginModel_PostProcessor::processExecutedMatch() was called then, a post processor was actually executed. This would also avoid the necessity of each implementation calling shouldExecute().