Add Sandcastle CI job and CMake build for BGP++ OSS

Summary:
Add the Sandcastle CI infrastructure and CMake build system for the BGP++ OSS build. This is the build/CI counterpart to the code-readiness changes in the parent diff.

Three areas:

1. **Sandcastle CI job** (`fboss/facebook/oss_ci/`):
   - `bgp_oss_build.bzl` — build macro adapted from `fboss_oss_build.bzl`. Key differences: source staging creates `bgp_src/` mirroring the BGP++ OSS layout, passes `bgp` as the getdeps manifest, no SAI/SDK steps.
   - `BUCK` — adds `bgp-oss-build-manual` rule (manual-only for initial testing), builds `bgp_bin` cmake target.
   - Minor `fboss_oss_build.bzl` fix for shared infrastructure.
   - `run-getdeps.py` — helper script for getdeps invocation.

2. **CMake build** (`public_tld/CMakeLists.txt`, `FBGenCMakeBuildInfo.cmake`):
   - Wires up all BGP++ source files into tiered CMake targets (routelib → common → config_policy → transport → nexthop → rib → adjrib → peer → service → binary).
   - `bgp_fsdb_syncer` target for FsdbSyncer with FSDB client library dependencies.
   - `bgp_fsdb_fib_watcher` target for FsdbFibWatcher.
   - FBOSS install wiring via `find_path`/`find_library` for headers and static libs (no fbossConfig.cmake).
   - Vendored openr thrift IDL targets (Network, Platform) and utility libs (NetworkUtil, SystemMetrics).
   - Vendored FBOSS utility libs (AlertLogger, LogThriftCall).
   - C++20 default, `IS_OSS` compile definition, thrift layer dependencies.

3. **getdeps manifest** (`opensource/fbcode_builder/manifests/bgp`):
   - Lists BGP++ OSS dependencies (folly, fb303, fbthrift, fizz, fmt, wangle, re2, zstd, gcc12).

Reviewed By: xiangxu1121

Differential Revision: D106590984

fbshipit-source-id: 488b257a674b40f1e9a0ebcbd33ead55e660d1ef

Author: induvsuresh

build/fbcode_builder/manifests/bgp

@@ -1,6 +1,6 @@
 [manifest]
 name = bgp
-fbsource_path = facebook/neteng/fboss/bgp
+fbsource_path = fbcode/neteng/fboss/bgp
 shipit_project = bgp
 shipit_fbcode_builder = true
 
@@ -19,13 +19,18 @@ boost
 fb303
 fboss
 fbthrift
+fizz
+fmt
 folly
 magic_enum
+wangle
 re2
+zstd
+gcc12
 
 [shipit.pathmap]
-fbcode/neteng/fboss/bgp = neteng/fboss/bgp
 fbcode/neteng/fboss/bgp/public_tld = .
+fbcode/neteng/fboss/bgp = neteng/fboss/bgp
 fbcode/fboss/common = common
 
 [shipit.strip]