Support azure Key Vault as an backend

[knelasevero]

Describe the solution you'd like Support Azure Key Vault as a backend for secrets.

What is the added value? Having new backends increases chances for adoption and broadens the operator use cases.

Give us examples of the outcome

apiVersion: store.externalsecret-operator.container-solutions.com/v1alpha1
kind: SecretStore
metadata:
  name: secretstore-sample
spec:
  controller: staging
  # Sample store types
  #
  # akv  
  store:
    type: akv
    auth: 
      secretRef: 
        name: externalsecret-operator-credentials-akv
        namespace: externalsecret-operator-system
    parameters:
#### TODO: not sure what parameters to add
#### ...

Observations (Constraints, Context, etc):

Also not sure bout this name "akv". Anybody that picks this can start the discussion or decide on a initial good name for it.