Create RoleFinder Interface

## Create RoleFinder Interface

### Acceptance Criteria
1. `\Synapse\User\RoleFinder` interface exists with a single public method: `findRoleNamesByUserId`.
1. `UserRolePivotMapper` implements the interface.

### Tasks
- None

### 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.