stats: add support for ring buffer type

[hsandid]

Hello all,

This PR adds support for ring buffers entries recently introduced as part of VPP stats-segment (https://gerrit.fd.io/r/c/vpp/+/43471).

This updates statseg_v2 to be able to fetch ring-buffer information (config, metadata), as well as return ring-buffer entries in raw bytes format. The stats-client can also be now be used to poll/dump ring-buffer information.

➜  bin/stats-client dump /test/ring-buffer
Dumping stats.. /test/ring-buffer
 - /test/ring-buffer                                               RingBuffer
  config: entry_size=258, ring_size=4096, threads=3, schema_version=1, schema_size=45
  thread[0]: head=29 seq=29 schema_version=1
  thread[1]: head=13 seq=13 schema_version=1
  thread[2]: head=8 seq=8 schema_version=1
Dumped 1 (1) stats