add support for description from file in topic resource - #52

On this PR, I added the possibility of using a separate file for the description of a topic resource.
For example, now the user can apply the following yaml:

---
apiVersion: v2
kind: Topic
metadata:
  cluster: local
  name: tata
  labels: {
    "conduktor.io/descriptionFile": "tata_desc.md"
  }
spec:
  replicationFactor: 1
  partitions: 3
  configs:
    min.insync.replicas: '1'
    cleanup.policy: delete
    retention.ms: '60000'

If tata_desc.md contain the following content:

This is a critical topic

This would be the equivalent of applying the following resource:

---
apiVersion: v2
kind: Topic
metadata:
  cluster: local
  name: tata
  labels: {
    "conduktor.io/description": "This is a critical topic"
  }
spec:
  replicationFactor: 1
  partitions: 3
  configs:
    min.insync.replicas: '1'
    cleanup.policy: delete
    retention.ms: '60000'

I took the opportunity to improve error handling of the similar existing mechanism for subject schema.