Enhance Integration with Spark

[levscaut]

Why are these changes needed?

As Spark DataFrames and a new model class, SparkLGBMEstimator, have been introduced in FLAML, there is currently no official documentation detailing their usage. I am in the process of adding the necessary documentation for these Spark-related features.

Additionally, I encountered minor issues with lgbm_spark while revising this model. Users were required to specify their label twice, in both the label and labelCol arguments. I've eliminated the need for input in labelCol and aligned it with label by default. This enhancement makes the Spark models more consistent with the FLAML style.

Related issue number

Closes #1088

Checks

• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[thinkall]

LGTM. Thanks.