[FEATURE REQUEST] onboard bucket via onyxia-operator

OnyxiaAPI should allow a configuration where S3 bucket with quota is provided by onyxia-operator :
As a first step we can start with :

- [ ] add a regional option for that
- [ ] generate CRD in a new API endpoint

We should take care about security issues.
At the moment there is no admin account of the object storage in Onyxia but the operator will.
So we need to be sure that the created bucket belong to the user.

In a second step we could implement policy generation. This will be done in another FR.