[Suggestion] Support flexible paths for secrets

Hello, thanks a lot for this plugin, many people are waiting for it (me too).
There is one problem of the plugin's design though.
If one start 'from scratch' - everything is fine, the secrets will be stored/retrieved at the paths you want:
```
/device/{id}/{slug} for Devices
/vm/{id}/{slug} for Virtual Machines
/service/{id}/{slug} for Services

"secret_path_prefix": "/netbox",
```

But what about the existing Vault installation, where many secrets are ALREADY stored, at some paths?
I think that this plugin should have a flexibility to configure the path for **every** secret.
It means that the user should be able not only _add_ a secret at the 'standard' path, but he should be able to _link_ an _existing secret_ at a custom path to a device/VM/service too.
It does not seem to be a really complex modification, but it should be done at the beginning of the development of the plugin, because IMHO it will be more difficult later.