Add generic type for layout variant

[maxiadlovskii]

Description

We making layoutVariant generic to be able define it from places where they might be different depending on usage

Motivation and Context

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 requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

/nocl
/jpd https://github.com/Graylog2/graylog-plugin-enterprise/pull/14542