Added implementation of the libp2p perf protocol for measuring transfer performance.

The perf protocol allows benchmarking data transfer speeds between libp2p nodes. It includes:
- ``PerfService`` class for perf protocol server/client operations
- ``measure_performance()`` method to measure upload/download throughput to a remote peer
- Server-side handling for responding to perf protocol requests
- Detailed metrics output including latency, upload/download times, and throughput in bytes/second

See the spec at https://github.com/libp2p/specs/blob/master/perf/perf.md