[IAM] Support multiple roles per binding

### TL;DR

e.g.:
bindings:
- members:
- 'user:xxx@xxx.net'
role:
- 'roles/compute.admin'
- 'roles/xxx'

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_

Look at the IAM binding structures in the codebase, likely in a package handling Google Cloud IAM policies. The change involves modifying the binding schema to accept a list of roles instead of a single role. Start by finding where bindings are defined and validated, then update the struct and any related serialization/deserialization logic. Check for existing tests around IAM bindings to understand the expected behavior.

Rédigé par le modèle d'indexation à partir du texte de l'issue.