Add optional support for building native extensions of path sources

[akiellor]

What was the end-user or developer problem that led to this PR?

Bundler did not build native extensions for gems sourced from a local :path, which is a common setup in monorepos. That meant path-based gems that relied on compiled code could not be used unless developers built those extensions manually in their checkout. In practice, this made local development in monorepos inconsistent with out-of-tree gems, broke bundle exec / require for those gems, and added manual setup steps that made the developer experience worse.

What is your fix for the problem, implemented in this PR?

This adds a new build_path_extensions setting for local path gems with native extensions. When enabled, Bundler compiles those extensions out of tree under BUNDLE_PATH, keeps the checkout untouched, and puts the built extension directory ahead of the gem's own load paths so bundle exec and require can load the compiled artifacts.

The feature supports per-gem opt in via build_path_extensions.<gem>, reuses existing build flags from build.<gem>, avoids rebuilding when nothing changed, and rebuilds when native extension sources or build flags change. It also surfaces native extension build output for path sources and preserves the existing failure mode when a required extension has not been built or has gone stale.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[akiellor]

This PR is an attempt to tackle this issue. Feedback is welcome 🙏.