Support specifying kubeconfig directly as yaml

[r4victor]

Closes #3167

It's now possible to specify kubeconfig for kubernetes backend directly as yaml via the data field:

type: kubernetes
kubeconfig:
  data:
    apiVersion: v1
    clusters:
    - cluster:
        # ...
    contexts:
    - context:
        # ...
    # ...

Previously, kubeconfig contents have to be converted to a string first.

[r4victor]

Closing as you can already specify kubeconfig as yaml multiline string, e.g.

type: kubernetes
kubeconfig:
  data: |
    apiVersion: v1
    clusters:
    - cluster:
        # ...
    contexts:
    - context:
        # ...
    # ...