[Nexthop][fboss2-dev] BGP-aware fboss2 config session (ConfigSession infra)

[hillol-nexthop]

Summary

Make the fboss2 ConfigSession and config session lifecycle BGP-aware. This
is the base for the config protocol bgp commands, which build on top (see the
stacked PR).

• ConfigSession owns the BGP config as a typed bgp::thrift::BgpConfig,
  exactly the way it owns cfg::AgentConfig for the agent.
  getBgpConfig()/saveBgpConfig() expose the whole typed config and stage it
  to ~/.fboss2/bgp_config.json. ConfigSession is BGP-aware but agnostic
  about the config's internal structure: a command mutates whichever typed
  fields it needs, mirroring how interface commands mutate
  getAgentConfig().sw()->ports(). The whole config round-trips through the
  typed struct, so no surgical merge / preserve-list is needed.
• commit() promotes bgp_config.json to /etc/coop/bgpcpp/bgpcpp.conf,
  restarts bgp_pp, and versions it in the same /etc/coop git repo as
  agent.conf.
• config session clear/diff/rebase/rollback are BGP-aware: clear removes a
  staged BGP session, diff shows staged BGP changes, rebase 3-way-merges
  bgpcpp.conf, rollback restores bgpcpp.conf + restarts bgp_pp (using
  metadata history so BGP-only commits are reachable).
• FbossServiceUtil + cli_metadata gain the bgp_pp (BGP_PP)
  service/action; the config lib gains the bgp_config cpp2 dependency.

Test Plan

• fboss/cli/fboss2/test/config:cmd_config_test — 455 tests pass
  (the BGP-session unit tests for the fixes below land in this PR).
• //fboss/cli/... builds on top of upstream/main.
• End-to-end coverage of the BGP session lifecycle on a live bgp_pp runs as
  ConfigBgpSessionTest, which is added in the stacked PR (the integration
  harness ships with the config protocol bgp commands) — 6/6 pass there.