add support for rest server backends

Backend configuration would be;

```
rest_repo:
  type: rest
  path: http://backup:8001/repo_name
```

Would result in the following env var;
```
RESTIC_REPOSITORY=rest:http://backup:8001/repo_name
```

src/backend.ts

@@ -17,9 +17,8 @@ export const getPathFromBackend = (backend: Backend): string => {
 		case 'gs':
 		case 's3':
 		case 'sftp':
-			return `${backend.type}:${backend.path}`
 		case 'rest':
-			throw new Error(`Unsupported backend type: "${backend.type}"`)
+			return `${backend.type}:${backend.path}`
 		default:
 			throw new Error(`Unknown backend type.`)
 	}