Custom pod annotations for Kubernetes runner

[LouisVN]

Allow conditional definition of custom annotations for the pod created by the Kubernetes runner such as:

  # Runner configuration
  config:
    # The kubernetes runner configuration
    kubernetes:
      # Kubernetes context to use
      context: mu-kube-context
      # Namespace to use
      namespace: env-staging
      # Annotations to attach to the pods 
      annotations:
        key1: value1
        key2: value2

This follows work around allows a new simple authentication and security layer (SASL) mechanism called AWS_MSK_IAM for the Amazon MSK service (see #56).
In some cases, it can notably be useful to define annotations in order to be able to assume IAM roles within Kubernetes, authorizing interaction with resources living in MSK.