add 'current_droplet' to app's relationships

[gmllt]

The only way to retrieve the current droplet of an application via the API is to go through the /v3/apps/:app_guid/droplets/current or /v3/apps/:app_guid/relationships/current_droplet endpoints.

In a context where we want to retrieve all the applications (more than 3000 in my case) and their current droplet, it is therefore necessary to make one request per application (more than 3000 requests).

This PR aims to make the current_droplet relationship visible on all endpoints presenting the app object :

• /v3/apps
• /v3/apps/:app_guid
• /v3/staks/:stack_id/apps
• ...

See issue #3860

• [x] I have reviewed the contributing guide

• [x] I have viewed, signed, and submitted the Contributor License Agreement

• [x] I have made this pull request to the main branch

• [x] I have run all the unit tests using bundle exec rake

• [x] I have run CF Acceptance Tests