feat(extension): support out-of-tree extension builds via NEUG_EXTENSION_CONFIGS

[liulx20]

Summary

Support building NeuG extensions outside the NeuG source tree (out-of-tree), so an extension repo can keep NeuG as a git submodule / sibling checkout and compile against it without copying sources into neug/extension/.

Motivation

Extension authors typically want a layout like:

my-extension/
  neug/                     # submodule or sibling
  extension_config.cmake
  CMakeLists.txt
  src/

Today, only in-tree extensions under extension/ (e.g. parquet, gds, pattern_matching, httpfs) are wired into the NeuG CMake build via BUILD_EXTENSIONS. External repos need a first-class registration hook.

Proposed design

1. Add a CMake cache variable NEUG_EXTENSION_CONFIGS: semicolon-separated list of extension_config.cmake files.
2. Provide cmake/neug_extension.cmake with neug_extension_load(<name> SOURCE_DIR <path>) to register an external extension source tree.
3. When configuring NeuG, include those config files; for each name in BUILD_EXTENSIONS:
   • builtins → add_subdirectory(extension/<name>)
   • registered external → add_subdirectory(<SOURCE_DIR>) into the NeuG build tree

Example:

# my-extension/extension_config.cmake
neug_extension_load(wiggle
    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}
)

cmake -S neug -B build \
  -DBUILD_EXTENSIONS=wiggle \
  -DNEUG_EXTENSION_CONFIGS=/path/to/my-extension/extension_config.cmake

Artifacts land under the NeuG build dir, e.g. build/extension/wiggle/libwiggle.neug_extension.

Implementation

Tracking PR: #745

[github-actions]

🤖 Auto-triage suggestion: this looks like a sub-issue of #257 — Grouping adhoc issues (confidence: medium).

Reason: This is a build-system/extension-infra enhancement for out-of-tree extension builds, which doesn't fit neatly into any specific umbrella's scope_in; it's a cross-cutting build infrastructure improvement that falls into the adhoc bucket.

Auto-applied: type → Feature; labels → ci, extension

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