Add an opt-in standalone build proxy launcher

[karim-alweheshy]

Summary

• Add the opt-in Xcode/xcodebuild launcher and install the exact proxy inside the generated project.
• Verify proxy, manifest, label, project identity, and Xcode build before selecting XCBBUILDSERVICE_PATH.
• Fail closed on partial configuration, digest drift, symbolic inputs, malformed hashing tools, or incompatible Xcode selection.

Stack

• Part of Feature Request: Have a build mode that leverages an XCBBuildService proxy for a more integrated experience #1953.
• 3 of 5 in the rules_xcodeproj build-proxy stack.
• Parent: the generated Bazel adapter PR.
• Next: configured Bazel action inventory.

Validation

• Repository CI is green at this exact head.
• //test/internal/build_proxy_launcher:build_proxy_launcher_tests passes at the combined head.
• Shell syntax, ShellCheck, Buildifier, and digest negative controls pass.

Testing

Configure build_proxy and build_proxy_sha256, regenerate the project, launch through rules_xcodeproj/build_proxy/launch_with_build_proxy.sh, and verify mapped builds use the proxy while an unconfigured project remains native.