feat(cluster/graph): support filtering by roots

Description

Add a flag to allow providing CEL expressions to determine root vertexes.

Demo: https://youtu.be/dUCmX8OtNm8

Related Issues

Relates to Implement roots filtering (gitlab-org/cluster-integration/gitlab-agent#703 - closed).

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)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap