feat: migration service api

## Is your feature request related to a problem? Please describe.
As a developer building a migration solution from other cloud services (like ownCloud 10) I'd like to have an api to impersonate any user in ocis.
Due to the nature of migrations at no time trough out the migration is running any user interaction is possible.

## Describe the solution you'd like
Within OCIS as an admin I can define a migration client which I can turn on/off and delete as I like.
The migrations API will basically hold exactly one API call:
```
POST /../exchangeToken
JSON-Body:
Some user identifier like email or uuid
```

The response will be a single access token which can be used to interact with ocis apis (mainly file upload and such via webdav as the real migration workhorse will be rclone and any api which will allow an impersonated admin account to create users)

For authentication of this single api call we will use a private/public key mechanism were the private key stays within the migration api client and the public key is shared with the ocis server.
E.g some JWT which is signed with the key material.

As soon as the migration is done the client registration can be removed.