Adds new preRegistrationScript to run commands before the registration happens

What does this MR do?

Adds new preRegistrationScript to run commands before the registration happens

Why was this MR needed?

When using a secret manager like vault that will directly inject tokens into the pod, such as the runner token. We want to be able to run some commands to change the CI_SERVER_TOKEN environment variable before the runner registration happens.

This also solves #462 (closed)

What's the best way to test this MR?

What are the relevant issue numbers?

values.yaml

@@ -61,8 +61,8 @@ imagePullPolicy: IfNotPresent
 ## be retrieved from your GitLab Instance. It is token of already registered runner.
 ## ref: (we don't yet have docs for that, but we want to use existing token)
 ##
-# runnerToken: ""
-#
+runnerToken: "placeholder"
+
 
 ## Unregister all runners before termination
 ##