Add support for Vault Password Policies in VaultStaticSecret

**Is your feature request related to a problem? Please describe.**

Hello, I use auto-generated passwords for some apps on Kubernetes, mostly for internal API tokens and session passwords.
However, these passwords are not synchronized to or from Vault (i use [mittwald/kubernetes-secret-generator](https://github.com/mittwald/kubernetes-secret-generator)).

I have two problems with theses password not synchronized to Vault:
- If I redeploy a cluster from scratch (e.g., after a disaster), my passwords and API tokens change.
- When I deploy two identical clusters on different sites, they do not have the same password/API token on both sides.

**Describe the solution you'd like**

I would like to have a **passwordPolicy** field on **VaultStaticSecret** that auto-generates a password and stores it back to Vault if the specified path is empty and the password policy exist. eg:

~~~
apiVersion: secrets.hashicorp.com/v1beta1
kind: VaultStaticSecret
metadata:
name: mysecret
spec:
mount: kv2
type: kv-v2
path: mysecret
passwordPolicy: mypolicy
refreshAfter: 60s
destination:
create: true
name: mysecret
~~~

If the path `kv2/mysecret` is empty and the password policy **mypolicy** exist, a password is generated from `/sys/policies/password/mypolicy/generate` and stored to `kv2/mysecret`.

**Describe alternatives you've considered**

I have considered to generate theses password from Terrafom (or other tools) and not directly from Kubernetes but that's broke my GitOps workflow and leak password to Terraform state.

Thank's,