[AURON #2020] [BUILD] Add --threads option to control Maven build parallelism.

[slfan1989]

Which issue does this PR close?

Closes #2020

Rationale for this change

Currently, local builds are always single-threaded, which can be slow on multi-core machines. Docker builds hardcode -T8, which cannot be overridden by users. This change adds a --threads option to auron-build.sh to give users control over Maven build parallelism.

What changes are included in this PR?

1. auron-build.sh

   • Added --threads parameter parsing and validation
   • Unified thread configuration logic: user-specified value takes precedence, Docker defaults to 8 threads, local defaults to single-threaded
   • Removed hardcoded -T8 from Docker build section
   • Updated help text to document the new option

2. CONTRIBUTING.md

   • Documented --threads option under Build Options section
   • Described default behavior for local vs Docker builds

Are there any user-facing changes?

Yes. Users can now specify --threads to control Maven build parallelism

Default behavior remains unchanged (backward compatible):

• Local builds: single-threaded
• Docker builds: 8 threads

How was this patch tested?

• Verified ./auron-build.sh --help displays correct usage information

[slfan1989]

./auron-build.sh --pre --sparkver 4.1 --scalaver 2.13 --threads 2

[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for Apache Auron Parent Pom 7.0.0-incubating:
[INFO] 
[INFO] Apache Auron Parent Pom ............................ SUCCESS [ 11.384 s]
[INFO] Apache Auron Proto ................................. SUCCESS [ 28.548 s]
[INFO] Apache Auron Hadoop Shim 2.13 ...................... SUCCESS [  8.290 s]
[INFO] Apache Auron Core .................................. SUCCESS [  9.705 s]
[INFO] Apache Auron Common 2.13 ........................... SUCCESS [ 15.309 s]
[INFO] Apache Auron Spark Version Annotation Macros 2.13 .. SUCCESS [  6.961 s]
[INFO] Apache Auron Spark UI 2.13 ......................... SUCCESS [  6.463 s]
[INFO] Apache Auron Spark Extension 2.13 .................. SUCCESS [ 35.552 s]
[INFO] Apache Auron Spark Extension Shims Spark 4.1.1_2.13  SUCCESS [ 28.957 s]
[INFO] Apache Auron Assembly .............................. SUCCESS [ 12.828 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  02:07 min (Wall Clock)
[INFO] Finished at: 2026-02-18T14:00:14+08:00
[INFO] ------------------------------------------------------------------------

./auron-build.sh --pre --sparkver 4.1 --scalaver 2.13 --threads 4

[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for Apache Auron Parent Pom 7.0.0-incubating:
[INFO] 
[INFO] Apache Auron Parent Pom ............................ SUCCESS [  6.504 s]
[INFO] Apache Auron Proto ................................. SUCCESS [ 25.801 s]
[INFO] Apache Auron Hadoop Shim 2.13 ...................... SUCCESS [ 10.265 s]
[INFO] Apache Auron Core .................................. SUCCESS [  8.988 s]
[INFO] Apache Auron Common 2.13 ........................... SUCCESS [ 14.699 s]
[INFO] Apache Auron Spark Version Annotation Macros 2.13 .. SUCCESS [  6.991 s]
[INFO] Apache Auron Spark UI 2.13 ......................... SUCCESS [  6.127 s]
[INFO] Apache Auron Spark Extension 2.13 .................. SUCCESS [ 26.319 s]
[INFO] Apache Auron Spark Extension Shims Spark 4.1.1_2.13  SUCCESS [ 25.859 s]
[INFO] Apache Auron Assembly .............................. SUCCESS [ 14.000 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:47 min (Wall Clock)
[INFO] Finished at: 2026-02-18T14:02:31+08:00
[INFO] ------------------------------------------------------------------------