[BUILD] Add `--threads` option to control Maven build parallelism

[slfan1989]

Description

Add a --threads option to auron-build.sh to allow users to configure Maven build parallelism for both local and Docker builds.

Motivation

Currently:

• Local builds are always single-threaded, which can be slow on multi-core machines
• Docker builds hardcode -T8, which cannot be overridden
• Users lack documentation about parallel build options

Changes

1. auron-build.sh

   • Added --threads parameter to control Maven -T option
   • Removed hardcoded -T8 from Docker build logic
   • Priority: user-specified > Docker default (8) > local default (single-threaded)
   • Updated help text with usage examples

2. CONTRIBUTING.md

   • Documented --threads option and default behavior

Usage Examples

• Local build with 4 threads

./auron-build.sh --pre --sparkver 3.5 --threads 4