* Bootstrapping initial support for Metadata Table in Spark Datasource
- Consolidated Avro/Row conversion utilities to center around Spark's AvroDeserializer ; removed duplication
- Bootstrapped HoodieBaseRelation
- Updated HoodieMergeOnReadRDD to be able to handle Metadata Table
- Modified MOR relations to be able to read different Base File formats (Parquet, HFile)
Currently, HadoopFsRelation will use the value of the real partition path as the value of the partition field. However, different from the normal table, Hudi will persist the partition value in the parquet file. And in some cases, it's different between the value of the real partition path and the value of the partition field.
So here we implement BaseFileOnlyViewRelation which lets Hudi manage its own relation.
* Introduce hudi-spark3-common and hudi-spark2-common modules to place classes that would be reused in different spark versions, also introduce hudi-spark3.1.x to support spark 3.1.x.
* Introduce hudi format under hudi-spark2, hudi-spark3, hudi-spark3.1.x modules and change the hudi format in original hudi-spark module to hudi_v1 format.
* Manually tested on Spark 3.1.2 and Spark 3.2.0 SQL.
* Added a README.md file under hudi-spark-datasource module.
