Refactor static cloud compatibility check for inputs

[patrickmann]

/nocl (bug in unreleased change)
/prd Graylog2/graylog-plugin-enterprise#7044

Relates to Graylog2/graylog-plugin-enterprise#7019

#18769 changed the signature of MessageInput.Descriptor methods to be static, to support pre-install compatibility checks for content packs. However, other parts of the code rely on dynamic overriding of these methods.

To support both use cases, we revert back to the original state and instead introduce a static check method, specifically for content pack imports in Cloud. All MessageInput subclasses that want to be cloud-compatible need to define this new static method in MessageInput.Descriptor.

How Has This Been Tested?

• Can create inputs in Cloud and on-prem
• Cloud only shows cloud-compatible inputs; on-prem shows all
• Cloud-incompatible inputs are ignored when importing a content pack in Cloud

Note: an exception is thrown when uninstalling a content pack. It is related to audit logging and is not relevant to this issue, since it reproduces on master. We should fix that separately (#19034).

2024-04-12 12:25:04,859 WARN : org.graylog.plugins.auditlog.jersey.AuditLogFilter - Couldn't capture response entity
java.lang.IllegalArgumentException: No serializer found for class

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.