add support for image override for argocd-extension

[anandrkskd]

What type of PR is this?

/kind enhancement

What does this PR do / why we need it:
This PR adds downstream support of argocd-extension in gitops-operator using env var
Have you updated the necessary documentation?

• Documentation update is required by this PR.
• Documentation has been updated.

Which issue(s) this PR fixes:

Fixes GITOPS-5661

How to test changes / Special notes to the reviewer:

[svghadi]

Looks like the new unit test is failing

--- FAIL: Test_getRolloutInitContainer (0.00s)
    --- FAIL: Test_getRolloutInitContainer/when_running_in_gitops#01 (0.00s)
        deployment_test.go:2448: 
            	Error Trace:	/home/runner/work/argocd-operator/argocd-operator/controllers/argocd/deployment_test.go:2448
            	Error:      	Not equal: 
            	            	expected: "quay.io/argoprojlabs/argocd-extension-installer:v0.0.8"
            	            	actual  : "updated_container"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-quay.io/argoprojlabs/argocd-extension-installer:v0.0.8
            	            	+updated_container
            	Test:       	Test_getRolloutInitContainer/when_running_in_gitops#01
            	Messages:   	Image check
        deployment_test.go:2449: 
            	Error Trace:	/home/runner/work/argocd-operator/argocd-operator/controllers/argocd/deployment_test.go:2449
            	Error:      	Not equal: 
            	            	expected: []v1.EnvVar{v1.EnvVar{Name:"EXTENSION_URL", Value:"https://github.com/argoproj-labs/rollout-extension/releases/download/v0.3.6/extension.tar", ValueFrom:(*v1.EnvVarSource)(nil)}}
            	            	actual  : []v1.EnvVar(nil)
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1,8 +1,2 @@
            	            	-([]v1.EnvVar) (len=1) {
            	            	- (v1.EnvVar) {
            	            	-  Name: (string) (len=13) "EXTENSION_URL",
            	            	-  Value: (string) (len=89) "https://github.com/argoproj-labs/rollout-extension/releases/download/v0.3.6/extension.tar",
            	            	-  ValueFrom: (*v1.EnvVarSource)(<nil>)
            	            	- }
            	            	-}
            	            	+([]v1.EnvVar) <nil>
            	            	 
            	Test:       	Test_getRolloutInitContainer/when_running_in_gitops#01
            	Messages:   	Env check

[svghadi]

Please fix the unit test. Rest all looks good.

[anandrkskd]

Hey Siddhesh, tests are passing now

[svghadi]

LGTM. Thanks.