Installed iceberg-config.cmake calls find_dependency(Avro) instead of avro-cpp

[rohanjain101]

Summary

When iceberg-cpp is built against a system Avro (avro-cpp found through FIND_PACKAGE_ARGS), the installed
iceberg-config.cmake can't be consumed: find_package(iceberg CONFIG) fails at find_dependency(Avro).

Cause

In resolve_avro_dependency() (cmake_modules/IcebergThirdpartyToolchain.cmake), the non-vendored branch does:

  else()
    set(AVRO_VENDORED FALSE)
    list(APPEND ICEBERG_SYSTEM_DEPENDENCIES Avro)
  endif()

However, the package that was actually found is avro-cpp. It is declared with fetchcontent_declare(avro-cpp ... FIND_PACKAGE_ARGS ...), and its config exports the avro-cpp::avrocpp_* targets. Avro 1.12 installs
avro-cpp-config.cmake and no Avro package. So iceberg_find_dependencies() in the installed config calls
find_dependency(Avro), which can't be satisfied.

Reproduction

Seen with v0.4.0 (the code is unchanged on main) using vcpkg's avro-cpp 1.12.2 on x64-windows:

1. Build and install iceberg-cpp with the bundle enabled against the system avro-cpp.
2. In a consumer project, call find_package(iceberg CONFIG REQUIRED).
3. Configuring fails because the Avro package can't be found.

Suggested fix

Record the real package name:

    list(APPEND ICEBERG_SYSTEM_DEPENDENCIES avro-cpp)

The same one-line change is carried as a patch in the proposed vcpkg port; I'm happy to open a PR here as well.