Add CallGraph query API

Despite the fact that a call graph can be built from a flame graph or a profile in pprof format, the new use-case specific API will significantly help us integrate call graphs into the Pyroscope UI.

The issue lies in the fact that truncating flame graphs/pprof profiles is not suitable for call graphs: the heuristics we use to trim insignificant nodes make the call graph incorrect. A workaround could be keeping all the nodes (disabling the truncation); however, this will not work well with large profiles. Thus, it's preferable to build call graphs in the backend, trimming nodes in a way optimized for call graphs; otherwise, we will be juggling between incorrect call graph structures/numbers and very poor performance.