Create RoleFinder Interface and Decorator

## Create RoleFinder Interface

### Acceptance Criteria
1. `\Synapse\User\RoleFinderInterface` interface exists with a single public method: `findRoleNamesByUserId`.
1. `UserRolePivotMapper` and `RoleService` implement the interface.
1. `RoleFinderDecorator` exists.
 - Constructor takes a single `RoleFinderInterface` to decorate.

### Tasks
- Create `RoleFinderInterface`
 - Make service and mapper implement it.
- Create `RoleFinderDecorator`

### Additional Notes
- This is helpful in case any transformation needs to happen to the roles after being retrieved from the database. (Adding "faux roles", adding a prefix, etc.)
- For transformations, create a decorator.