Provide API to access span IDs in profiles

Currently there is no way of accessing the SpanIDs which are existing in Pyroscope profile table. We should provide an endpoint that can list span IDs for a given profile series label selector and time window. It also should optionally accept filtering for span IDs, to test their existence and total value.

```
// Ability to list span ids by profile series selector, and optionally a list of spanIDs
message ListSpanIDRequest {
repeated string matchers = 1;
// Milliseconds since epoch. If missing or zero, only the ingesters will be
// queried.
int64 start = 2;
// Milliseconds since epoch. If missing or zero, only the ingesters will be
// queried.
int64 end = 3;
// if given restrict to those span IDs
repeated string span_ids = 4;
// TODO: Pagination
}

message ListSpanIDsResponse {
repeated string span_ids = 1;
repeated string total_values = 2;
}
```

This could provde a good integration into getting extra span information from Tempo.