fix: add platform link deps for Arrow bundled S3 libraries in CMake config

[smaheshwar-pltr]

Issue

Closes #9

When ICEBERG_S3=ON with vendored Arrow (the default), arrow_bundled_dependencies contains AWS SDK C libraries that depend on platform system libraries. The installed iceberg-config.cmake creates the Arrow::arrow_bundled_dependencies IMPORTED STATIC target but doesn't declare these transitive dependencies.

Consumers using iceberg-cpp as a statically-linked library via find_package(iceberg CONFIG) with S3 support enabled hit linker failures:

Undefined symbols for architecture arm64:
  "_CFRelease", referenced from: ... in libarrow_bundled_dependencies.a(...)
  "_nw_connection_create", referenced from: ... in libarrow_bundled_dependencies.a[289](nw_socket.c.o)
  "_sec_identity_create", referenced from: ... in libarrow_bundled_dependencies.a[287](darwin_pki_utils.c.o)
  ... (30+ more _nw_*, _sec_*, _CF* symbols)
ld: symbol(s) not found for architecture arm64

PR

• When ICEBERG_S3=ON with vendored Arrow (the default), arrow_bundled_dependencies contains AWS SDK C libraries (aws-c-common, etc.) that depend on platform system libraries. The installed iceberg-config.cmake creates the Arrow::arrow_bundled_dependencies IMPORTED STATIC target but doesn't declare these transitive deps, causing unresolved symbols for downstream consumers at link time.
• Propagates ICEBERG_S3 into the config file via @ICEBERG_S3@ substitution and conditionally adds find_library + target_link_libraries(INTERFACE) for macOS frameworks (CoreFoundation, Security, Network) and Windows system libs (winhttp, bcrypt, etc.).
• Non-S3 builds are completely unaffected since the block is gated on if(ICEBERG_S3).

Details

Arrow's own ArrowConfig.cmake.in handles this same problem by checking "aws-c-common" IN_LIST ARROW_BUNDLED_STATIC_LIBS. Since iceberg-cpp's config doesn't have access to ARROW_BUNDLED_STATIC_LIBS, we gate on ICEBERG_S3 instead (which is the correct semantic equivalent -- S3 support is what pulls in the AWS SDK).

The find_library variable names (CORE_FOUNDATION, SECURITY, NETWORK) match Arrow's naming convention.

Test plan

• Build with ICEBERG_S3=ON, install, and verify the generated iceberg-config.cmake contains the platform-specific find_library and target_link_libraries calls
• Build with ICEBERG_S3=OFF (default) and verify the generated config has no platform-specific link libraries
• Link a downstream project against the installed iceberg package on macOS with S3 enabled -- should resolve all symbols without manual -framework flags