Add cluster agent customization validation tests

[a-blender]

Issue:

#41035

Problem

Solution

Test design (in progress)

• Add framework/extensions/agent/agentcustomization.go with AgentDeploymentCustomization test struct
• Add structs to config.go for cluster and fleet agent
• Write config yaml for cluster agent
• Add test for cluster agent (the move here is to add to existing provisioning v2 tests with single setup for rke1 and rke2. custom cluster? eks?)

Testing

Engineering Testing

Manual Testing

Automated Testing

Run the following to run my added tests

export CATTLE_TEST_CONFIG=./agent-customization-test.yaml
go test

QA Testing Considerations

Regressions Considerations

[a-blender]

Duplicate validation test PR, not needed. Unit tests were written by dev