Allow filtering on specific steps so that when using as a service we can trigger specific steps independently

https://github.com/google/slo-generator/blob/dcd2966ed787c925ee7f82ac1e2fe9113a3256d4/slo_generator/compute.py#L67

```
def compute(..., filter_steps=[]):
...
steps = error_budget_policy['steps']
if filter_steps: # filter on input step names
steps= [step for step in steps if step['name'] in filter_steps]
```