Export by matching label expressions

[jewertow]

Enable users to define service exports by using matchExpressions rules.

Users can use operators such as In, NotIn, Exists, and DoesNotExist for more fine-grained control as to which services should be exported.

Example structure

serviceSelectors:
  - matchExpressions:
      - key: app.kubernetes.io/name
        operator: In
        values:
          - ratings
          - reviews

[bartoszmajsak]

Took a liberty to reword the issue and link it to the part #142