Moving frontend build to process-classes phase.

[dennisoelkers]

Description

Motivation and Context

This PR is moving the frontend build steps for plugins from the compile phase to the process-classes phase. This is required together with #9229 to move the API generation step to a separate module that does not pull in test dependencies if run during the compile step.

/nocl No user-facing change.
/prd Graylog2/graylog-plugin-enterprise#9229

How Has This Been Tested?

Screenshots (if appropriate):

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.