Add the possibility to override the runner with initContainers / extra containers

[fdebuire]

Hello, with this doc it's possible to override part of the runner: https://docs.burrito.tf/user-guide/override-runner/

Use case context:
We have a use case where we need to run extra steps before running Terraform operations, basically our Terraform config is in yaml that includes some Jinja which need to be parsed before we can run the Terraform operations.

Maybe the solution would be to follow gitops principle and push this parsed configuration in the git repo before letting burrito handle the Terraform.

Feature request:
But we are also thinking an initContainer could solve this, and overall it might be useful to be able to override those specs.

[corrieriluca]

Hello,

Yes I think adding the possibility to specify init containers can be overall useful to many users.

There is also a use-case of retrieving ephemeral data (such as credentials from a custom enterprise solution) and storing them in a emptyDir volume shared with the burrito runner main container.

The implementation is straightforward: it's an addition to the OverrideRunnerSpec struct with a field to specify init containers. Kubernetes will handle the rest once the Pod is created by Burrito.

Edit: implementation can follow the same pattern as #530