[FEATURE] Multi instance controller deployment

## Is your feature request related to a problem? Please describe.
Right now Buildkite controller is single pod deployment. During DRT of our infrastructure one of datacenters was blackholed so it couldn't talk to the internet. This datacenter included controler which mean queues were not working and controller pod was not rescheduled. Having second controller on stand-by or in active-active mode would really help in such situations. After initial discussions with Buildkite support it seems after migration to Stacks API active-active setup might be feasible.

## Describe the solution you'd like
Ability in Helm chart to specify replicas of the controller and `topologySpreadConstraints` would be useful.

## Describe alternatives you've considered
We are looking into applying it via kustomize patch, parameter in Helm chart still would be more convenient.

## Additional context
n/a