feat: introduce HugeGraphSparkLoader

[simon824]

closed #281
basic version of HugeGraphSparkLoader

test cmd
The options are divided into two parts, spark options and hugegraph-loader options. (no sorting required)

 ./bin/hugegraph-spark-loader.sh  \
--master yarn --deploy-mode client --name spark-test \
--conf spark.eventLog.enabled=false \
--conf spark.executor.extraJavaOptions=-XX:+PrintGCDetails\
-f ./conf/spark.json --username admin --token admin \
-h 127.0.0.1 -p 8093 -g my_graph2 

[codecov]

Codecov Report

Merging #282 (ba7dedf) into master (7c39453) will decrease coverage by 4.61%.
The diff coverage is 1.72%.

@@             Coverage Diff              @@
##             master     #282      +/-   ##
============================================
- Coverage     75.74%   71.12%   -4.62%     
+ Complexity     1847      877     -970     
============================================
  Files           235       82     -153     
  Lines          7766     3816    -3950     
  Branches        685      456     -229     
============================================
- Hits           5882     2714    -3168     
+ Misses         1501      898     -603     
+ Partials        383      204     -179     

Impacted Files	Coverage Δ
...m/baidu/hugegraph/loader/executor/LoadContext.java	67.90% <0.00%> (ø)
...om/baidu/hugegraph/loader/mapping/InputStruct.java	93.02% <ø> (ø)
.../baidu/hugegraph/loader/source/AbstractSource.java	83.33% <ø> (ø)
...baidu/hugegraph/loader/source/file/FileFilter.java	75.00% <ø> (ø)
...aidu/hugegraph/loader/source/file/SkippedLine.java	91.66% <ø> (ø)
...u/hugegraph/loader/spark/HugeGraphSparkLoader.java	0.00% <0.00%> (ø)
...baidu/hugegraph/loader/mapping/ElementMapping.java	87.83% <50.00%> (-1.06%)	⬇️
...m/baidu/hugegraph/loader/executor/LoadOptions.java	74.41% <100.00%> (ø)
.../hugegraph/structure/graph/BatchVertexRequest.java
.../baidu/hugegraph/structure/schema/SchemaLabel.java
... and 152 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7c39453...ba7dedf. Read the comment docs.

[javeme]

Thanks for the contribution, it's a very nice enhancement. Some tiny comments

[imbajin]

could enable check-style plugin to check code

[javeme]

LGTM