Implement spark fanout writer

[XuQianJin-Stars]

Changes involved in this PR:

1. Move the PartitionedFanoutWriter of flink to core/src/main/java/org/apache/iceberg/io package.
2. Implement immobile writers for spark2 and spark3 modules respectively.
3. User can turn it on according to the parameter write.partitioned.fanout.enabled= true in the setting table. The default is false and can also set spark option partitioned.fanout.enabled to override it.

[HeartSaVioR]

Could you please elaborate this with details?

[XuQianJin-Stars]

Could you please elaborate this with details?

hi @HeartSaVioR well, I will verify the implementation of this function to supplement this isuses.

[HeartSaVioR]

I meant, what is the use case, and what is the goal of the issue, and what would be your approach to deal with this if you'd like to work on this. I guess this is essential information to construct as rationalization when adding a new feature.

[XuQianJin-Stars]

hi @HeartSaVioR This the PR, When you have time, please help me see this PR implementation.