add support for description from file in topic resource

[strokyl]

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.

[qboileau]

LGTM

@twobeeb BTW why we never went with the de-facto yaml standard !include 'xx.txt'.
Not in real yaml spec but implemented in some parsers and many people are familiar with it

[strokyl]

LGTM

@twobeeb BTW why we never went with the de-facto yaml standard !include 'xx.txt'. Not in real yaml spec but implemented in some parsers and many people are familiar with it

Quentin did you check if it work with the go parser?