Add support for service account creation

This MR should fix #159 (closed)

The auto-deploy-app chart actually already has a field to specify a service account name (serviceAccountName). But, it does not allow for the creation of the service account or adding annotations to it. While in theory it is possible to externally create the service account and then reference it with serviceAccountName, that is not practical when using Auto DevOps as the service account would need to be created within the namespace of each review environment, for instance.

We have internally used it by using a custom chart in the root folder of our repo, and it allowed us to create a service account during the deploy stage in Auto DevOps (as required to associate IAM roles to kubernetes deployments; see: https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html).

I can add some tests if you think the idea and design makes sense.