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?