fix: handle _json_key .dockerconfigjson data

[csatib02]

Overview

• Made the Auth field of type interface { }, because this way the program can handle a _json_key if it is received under the auth key of a .dockerconfigjson.
• I left the username, password splitting case as it was, and added a check for _json_key and another check, if a vault path is received upfront at the auth key.
• If we have sub-keys under the auth field, we can assume that a _json_key has been passed.
• if a vault path is passed upfront I used a regex to verify it's format.

Fixes #81

Notes for reviewers

• I tested the solution with these 3 dockerconfigjson secrets.

# test.yaml
# For this to work, run: vault kv put secret/test/mysql MYSQL_PASSWORD=3xtr3ms3cr3t
# Decoded structure of data:
# {
#   "auths": {
#     "https://myrepov": {
#       "auth": "vault:secret/data/test/mysql#MYSQL_PASSWORD"
#     }
#   }
# }

apiVersion: v1
kind: Secret
type: kubernetes.io/dockerconfigjson
metadata:
  name: broken-thing
  annotations:
    vault.security.banzaicloud.io/vault-skip-verify: "true"
data:
  .dockerconfigjson: eyJhdXRocyI6eyJodHRwczovL215cmVwb3YiOnsiYXV0aCI6ImRtRjFiSFE2YzJWamNtVjBMMlJoZEdFdmRHVnpkQzl0ZVhOeGJDTk5XVk5SVEY5UVFWTlRWMDlTUkE9PSJ9fX0=

# test2.yaml
# Decoded structure of data:
# {
#   "auths": {
#     "https://myrepov": {
#       "auth": {
#         "type": "service_account",
#         "project_id": "fake-project"
#       }
#     }
#   }
# }

apiVersion: v1
kind: Secret
type: kubernetes.io/dockerconfigjson
metadata:
  name: docker-secret
  annotations:
    vault.security.banzaicloud.io/vault-skip-verify: "true"
data:
  .dockerconfigjson: eyJhdXRocyI6eyJodHRwczovL215cmVwb3YiOnsiYXV0aCI6eyJ0eXBlIjoic2VydmljZV9hY2NvdW50IiwicHJvamVjdF9pZCI6ImZha2UtcHJvamVjdCJ9fX19

# test3.yaml
# For this to work, run: vault kv put secret/test/aws AWS_SECRET_ACCESS_KEY=s3cr3t
# Decoded structure:
# {
#   "auths": {
#     "https://myrepov": {
#       "auth": "vault:secret/data/test/aws#AWS_SECRET_ACCESS_KEY"
#     }
#   }
# }

apiVersion: v1
kind: Secret
type: kubernetes.io/dockerconfigjson
metadata:
  name: aws-key-secret
  annotations:
    vault.security.banzaicloud.io/vault-skip-verify: "true"
data:
  .dockerconfigjson: eyJhdXRocyI6eyJodHRwczovL215cmVwb3YiOnsiYXV0aCI6ImRtRjFiSFE2YzJWamNtVjBMMlJoZEdFdmRHVnpkQzloZDNNalFWZFRYMU5GUTFKRlZGOUJRME5GVTFOZlMwVloifX19