Add global customizer and filters

[christophejan]

• Customizer for all OpenAPI descriptions ("default" and GroupedOpenApi) #1609

Add GlobalOpenApiCustomiser GlobalOperationCustomizer and GlobalOpenApiMethodFilter that will be applied to :

1. Default OpenAPI description
2. GroupedOpenApi declared through spring properties (yml)
3. GroupedOpenApi declared programmatically like :

@Bean
public GroupedOpenApi mygroup(GroupedOpenApi.Builder builder) {
    return builder
        .group("mygroup")
        [...]
        .build();
}

The PR is composed by 4 commits that are more easy to review one by one. Only the last one add global customizer/filter.

Here some details on the commits :

1. Extra assertions on default group (on current behavior) :
   I would warn about a preexisting suspicious behavior when springdoc.use-management-port=true, querying nonexistent groups return a 404 with Webflux but a 500 with MVC.
2. Declare the most specific return type possible on some bean declaration :
   This is safer for components that implement several interfaces or for components potentially referred to by their implementation type (see Declaring a bean tip on spring core documentation)
3. Interact only with bean definition during bean registration phase :
   This will prevent any premature bean instantiation, violating the container and causing unintended side-effects (see BeanFactoryPostProcessor documentation). In concrete terms :
   • register bean definition rather than bean instance
   • do not require GroupedOpenApi beans instances anymore for SpringdocActuatorBeanFactoryConfigurer bean registrations
4. Add global customizers and filters

[bnasslahsen]

@christophejan,

Thank you for your contribution to the project.
If you have any other technical code enhancements, please feel free to propose a separate PR.

[christophejan]

@bnasslahsen, thank you for all your hard work on Springdoc