Move getGrantsForTarget method to helper class

[kingzacko1]

Description

Refactors getGrantsForTarget from EntityRegistrar to more appropriate EntitySharesService.
/nocl internal refactoring
/prd Graylog2/graylog-plugin-enterprise#14105

Motivation and Context

Needed to use the method for feature work, saw the TODO, figured I'd knock it out.

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.