Expose extraTemplateSpec for deployment-api.yaml

We've run into an issue with self-hosted Flagsmith where its pods are occasionally cycled due to other pods hogging resources. To resolve this we need to set a `priorityClassName` on the Flagsmith pods. This can be done through `deployment-api.yaml` through `spec.template.spec`. However, there is no hook in the chart to do so.

Recommendation: Add `extraTemplateSpec` to allow adding addition fields for the template in the deployment. This would allow us to specify `priorityClassName` and be flexible enough to handle other needs. This follows the pattern of `extraSpec` elsewhere.

Workarounds: Use `kustomize` or other means to modify the charts.