[FEATURE] Pod spec templating vars

## Is your feature request related to a problem? Please describe.

I'd like to use some buildkite variables when generating pod spec for the job. E.g. add `BUILDKITE_REPO` value in `pod.spec.volumes.csi.volumeAttributes`.

The controller adds `BUILDKITE_REPO` to the job and pod annotations. `pod-spec-patch` allows to modify the pod spec. But strategic merge patch does not allow to reuse field values from source.

## Describe the solution you'd like
I'd like the controller to provide templating functionality to generate the final job/pod spec, that supports embedding buildkite variables to the final spec.

## Describe alternatives you've considered
Defining individual pod templates per repository. This complicates a new pipeline creation process as now I also need to create a custom pod template.

## Additional context
N/A